This happens because the temperature at which oxygen changes from a gas to a liquid is about 13 degrees Celsius above that of nitrogen. The oxygen can be seen dripping from the can just as on a humid day you can see drops of water condense and drip from a can you take from the refrigerator.
Though you technically can join the military up to age 42, if you're 35 or over you'll need an age waiver if you've never been in the military before.
To join as a regular soldier you need to be at least 16 years old, although you can start the application process earlier, with your parents' permission. If you're under 18, you'll also need parental consent to join. You should be in Army phase 1 training before your 33rd birthday. To apply to become a regular officer you need to be between 18 - 28 years and 11 months, although you can apply for sixth form and university sponsorship before you reach this age.