How Old Do You Think You Are?

How Old Do You Think You Are?

You may think you know how old you are. But I'm about to tell you that you're wrong. You're wrong. There, see? I told you.

The Problems of Time and Identity

Time is a man-made construct. We created time as a standardised measurement to more easily describe and synchronise our lives. And yet our man-made clocks keep falling out of sync with natural events.

We define a day as 24 hours, but in the early Carboniferous period some 350 million years ago, Earth's faster spin meant a day lasted less than 23 hours. It took our planet 385 days to orbit the sun, which was a crying shame for dinosaurs, who got to celebrate far fewer birthdays.

Isn't there some natural universal clock that remains constant? Some measure of time that won't keep jumping around and giving palaeontologists headaches?

Nope. There's no ticking clock in nature. Even Einstein's model of space-time is relative; it changes depending on your speed or nearby masses.

Nature only cares for the laws of entropy. Entropy describes the degree of disorder in a system, operating in one direction: forwards. Physicists call this the arrow of time. But as far as nature is concerned, there is no standardised measure of time. The universe just doesn't work like that.

Here's another man-made construct: your identity. At any point in time (gah) there's no precise definition of you in the universe. You could argue that you are the sum of your cells, your organs, your brain, and your consciousness. But your bodily components are also subject to continual change.

That apple you ate for breakfast wasn't part of your body—but it is now. The glucose molecules of the apple were integrated into your cells and converted to energy. So now you're you plus an apple.

We started with a simple question: how old do you think you are? But as we unpack notions of time and identity, the question only begets more questions of physics, chemistry, biology, psychology, and philosophy, with countless moving parts as our scientific knowledge expands.

This post is an exploratory jab at some of the most intriguing answers about your true age at the smallest and the grandest scheme of things. It's an effort to expand your fundamental sense of self, according to four perspectives in science that produce startlingly different conclusions.

Take a deep breath. We're going in.

Biology Says You're As Old As Your Mum

You might think your life began when you burst out of the womb and took your first breath. But of course, you were alive inside the womb too. As a foetus, you had hiccups. You kicked your mum. You dreamed. So add a further eight months to your age, because that's when your heart actually started beating.

But was your first heartbeat really the beginning of you? What about the sperm and egg cells that fused to form your first complete set of DNA? Perhaps your life began at your conception. Or perhaps your life began at the inception of those fundamental sperm and egg cells.

Sperm cells are pretty short-lived. Sperm production takes about 64 days, and after sex they live for an average of just three more days. It's somewhat of a bang and a whimper. But egg cells live much longer.

Your mum was born with 7 million eggs in her ovaries. These eggs died off rapidly, and by puberty she had only 700,000 eggs left. One of those eggs was you.

This means that half of your genetic material was produced when your mum herself was a foetus developing inside your grandma.

Cartoon illustration of eggs developing in a female foetus

The egg that made you came from grandma

This yolky leap of logic means that half of you is actually as old as your mother. That adds another 20 or 30 years to your age, depending on the age of your mum. Don't worry though, you don't look a day over it.

Psychology Says You're Thirty Seconds Old

Here's a bit of psychology for you. You're not the same person you were a year ago, a week ago, a day ago—or even thirty seconds ago.

You experience personal identity as a steady condition. So steady, in fact, that you're not aware of the subtle incremental changes to your psychology that evolve over time. There was no perceptible change at the stroke of midnight when you turned 18 and were catapulted into adulthood. Yet, over some longer and hazier transitional period, you did progress through various physical and mental state changes.

Even profound moments in life are diffused over time to change your sense of self in imperceptible increments. It takes a few seconds to learn of a breakup or loved one's demise, but months or years to work through the grief that ultimately changes you as a person. Likewise, becoming depressed or an alcoholic is a slow burning process that becomes clinical significant long before you realise it.

Your sense of self is in a constant and unending state of flux.

Identity isn't defined by a series of boxes that are checked off from one day to the next, providing definitive borders to who you are. It's a series of overlapping experiences with fuzzy boundaries, an evolving mash-up of perceptions that are integrated only gradually.

But how does this make you thirty seconds old? This perspective comes down to working memory.

Your brain has the capacity to store incoming sensory information for about 20-30 seconds in short term (working) memory. It allows you to follow a conversation or type out a phone number. After 30 seconds, sensory input fades from awareness or transferred to long term memory. It's a bit like RAM, if you prefer your analogies digital.

Diagram how how short and long term memory works

Sensory information can be organised into different types of memories

This 30-second RAM may be the narrowest band of time we can attribute to your psychological sense of self, before it's scattered over numerous emotional and cognitive dimensions in transition. You may feel an ongoing sense of self lasting for years, but this feeling is a cognitive illusion, simulated by higher functioning parts of your brain to make sense of reality.

Consider the philosophical tale about the Ship of Theseus.

You know Theseus; he was the founder of Athens. And two thousand years ago, Plutarch the philosopher described how Theseus' long-abandoned ship decayed over time. This prompted the Athenians to replace it plank-by-plank until, after thirty years, it comprised none of the original wood.

Illustration of the philosophical thought experiment on the Ship of Theseus

Here's the paradox. The ship was qualitatively different—the wood had all new characteristics. And it was quantitatively different too—the ship was the sum of all new parts. Yet there was no definitive point in time when it stopped being the Ship of Theseus.

The same paradox can be applied to your identity. Every thirty seconds, a new plank is laid down, until you look back ten years and realise you're made of all new planks. But when did you become different?

Heraclitus nailed this 500 years before Plutarch, with a little less logic and a little more poetry.


It's an important point of philosophy, amassed from logic. Your identity evolves along a curve, a series of incremental deviations, such that any continuity of self is merely an illusion. You are thirty seconds old.

Quantum Physics Says You're Instantaneous

If thirty seconds sounded short-lived then this perspective will make you sick.

Let's say you define your identity not in a psychological sense, but in a physical one. Applying the Ship of Theseus logic to the smallest physical scale, we can strip down your thirty-second state of being to that of an instant.

Here's how it happens. We start with somewhat familiar building blocks—your cells—most of which have been replaced many times since the day you were born. Interestingly, this rate of replacement varies a fair bit between cell types:

Cartoon illustration of the age of different types of body cells

Different cell types have wildly different lifespans

Your body maintains itself by destroying and replacing 200 different types of cells on a continual basis. This is what led to the popular idea that the human body regenerates every 7-10 years. That's the average lifespan across all the different cell types.

If you identify as your sperm, you have a measly lifespan of three days. Not that anyone identifies as their sperm. I'm just saying.

Now let's dive deeper. Events inside your cells frequently change the qualitative and quantitative nature of your physical being. For instance, insulin is produced and destroyed in the space of just 5-6 minutes. Such hormones die very young indeed. But which processes are instantaneous?

We need to look at ATP.

ATP is the molecule you first learned about in high school biology. You hated it instantly because it meant memorising the godforsaken electron transport chain that produces it.

Assuming you didn't repress the memory, you'll remember that ATP is critical for carrying energy in cells. Without it you wouldn't be alive.

The ATP molecule contains a string of three phosphates. When phosphates are cut loose, energy is released into the cell. So far, so good. But now we must delve from biochemistry into quantum physics, something that gives me heart palpitations and I'm not sure if that's a good thing.

The nucleus of each phosphorus atom has a quantum property called spin. And it's interactions between these quantum spins that cause phosphates to become entangled.

Quantum entanglement of phosphates in the human body

Quantum entanglement of phosphates happens all the time in your body

Ah, quantum entanglement. What a devil. Einstein didn't like it; in fact he pointed to it as an example of just how little we then knew about quantum mechanics. Yet here we are today, getting on for a century later, using applied quantum entanglement in computers. The devil is real, even if his existence still makes no sense.

The thing that bothered Einstein is that particles remain entangled after they've been separated. In other words, quantum particles can become invisibly connected and "talk" to each other instantaneously, even when separated by vast distances. Einstein derided this as "spooky action at a distance".

Quantum entanglement cartoon: communicating faster than the speed of light

We still don't understand the mechanism for quantum entanglement.

Quantum entanglement is happening right now inside your body. Entangled phosphates are broken apart by enzymes but keep on chatting to each other, right across your brain, without any known method of doing so.

To put that another way, the entangled phosphates are suspended in quantum superposition. While phosphates incorporated into larger molecules could stay this way for a day or more, when the superposition does collapse, it happens instantaneously.

Say that again. Instantaneously. That's how fast quantum changes take place inside your body. That's how fast you change on the smallest physical scale. To put a figure on it, experiments with entangled photon pairs suggest the collapse occurs at one million times the speed of light.

I told you Einstein hated it.

Evolution Says You're 3.5 Billion Years Old

That was weird. I didn't really enjoy reducing your existence to a speck of an instant and, I suspect, neither did you. So here's an idea that extends your existence considerably, by some 3.5 billion years to be not-very-precise.

Remember when I said that half of your DNA was made inside your grandma in some crazy Russian doll biology? At what point did the amino acids, sugars, and phosphates that make up your DNA become "yours" as opposed to that of your foetal mother's, or even your grandmother's?

Moreover, where did the precise functional sequence of your DNA originate?

This perspective from evolutionary biology suggests you are much older than the genetic blueprint produced at your conception. The code that describes you as a living organism didn't pop out of nowhere—it came from a long and unbroken line of ancient ancestors.

Based on the earliest finds of Homo sapiens skeletons in Africa, our species is at least 200,000 years old. But a Homo erectus woman didn't give birth to a Homo sapiens infant and produce an entire new species in a single day. The genetic transition between species was, once again, a Ship of Theseus situation.

Evolutionary trees don't reveal this because they're simplifications of massive intergenerational complexity:

Illustrated timeline of human evolution

As Homo sapiens we're the only surviving species of hominins

Genetic analysis shows that the divergence between modern humans and our chimpanzee cousins varies greatly across the genome, suggesting long periods of hybridisation between our ancestral species. In hominids, this occurred five million years ago. It produced intermediate species between apes and humans called australopithecines. And so, once again, there's no definitive time we can say humans began.

Your genome isn't bordered by a single generation, a single species, or even a single order. It all blends together, bringing us to the mind-blowing conclusion that your DNA has a history of 3.5 billion years.

Illustration of the evolution of humans from bacteria

Your ancestry goes back billions of years to the origin of life itself

Your genome originated in the form of a simple bacteria billions of years ago, fossil evidence of which exists today in hot spring deposits in the Pilbara of Western Australia. The very source of your existence—replicated, exchanged, and expressed over countless generations—dates back 3.5 billion years.

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Becky Casale Author Bio


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