It was generally believed "bolting" was caused by higher temperatures, but studies have shown that temperature is not the only factor.
Bolting is also influenced by light exposure, water stress, and genetics.
When conditions are right, a lettuce plant will "bolt" to create rosette leaves and a flower stalk in order to reproduce.
This heading growth causes the lettuce to become bitter by producing compounds like acetic acid (vinegar) and worse to provide protection against insects.
That's not what you want in a lettuce plant.
You can certainly pick the lettuce before it has reached full size ( before the premature bolting), but you can also avoid the factors that cause the bolting in the first place.
There's not much you can do about the total day length or the temperature (besides planting earlier in the season), but Florida farmers around here have had encouraging results from growing lettuce under screening, which seems to (1) reduce the temperature and (2) fool the internal clock mechanism that causes lettuce to determine that it is receiving a lot of sun.
Try reducing the number of light hours by planting earlier in the season (or possibly by screening to see if that works).
Be sure to minimize moisture stress, particularly during the hotter months.
You might try some slow-bolting varieties.
You might have to try different varieties in your own garden to see how each resists bolting under your specific growing conditions; Results will vary.
gardening.stackexchange.com/users/85/Robert Cartaino
Lettuce gets bitter when it bolts.
See Robert Cartaino's answer for why this happens.
If you're going to pick baby greens you'll probably be able to harvest while it is still tasty.
Give your lettuce a little shade in hot weather to avoid wilting, and make sure it gets enough water.
If you're not bent on lettuce and just looking for some greens, consider chard or New Zealand spinach.
Chard does well for me in hot weather; picked at baby size it is a fine replacement for lettuce.
I haven't done NZ spinach, but references I have read recommend it as a green for hot summers.
gardening.stackexchange.com/users/51/bstpierre
Many plants produce bitter (or otherwise nasty) chemicals in defense of being eaten by insects .
Much of the reason for planting lettuce in cooler months is to give them a chance to reach maturity (for human consumption) before they've produced chemical weapons to shoo off the bugs.
gardening.stackexchange.com/users/47/Tangurena
Lettuce likes cool weather and lots of moisture, but there is no reason why you shouldn't sow or plant it out when the weather is hot.
Provided you give it the proper, weed-free spacing and keep it well-watered
, it is unlikely to bolt, even when the temperature is high.
That said, if you are planting out seedlings, it is probably better to do so in cool, preferably cloudy weather, or in the evening, to prevent wilting; it helps if you can give them some sort of makeshift shade for a few days.
I grow mine in light, dappled shade, and find that they taste best when grown as fast as possible - they therefore need plenty of food and water.
gardening.stackexchange.com/users/136/Mancuniensis
Various sources will say that cool weather is favoured for a sweet, tender lettuce crop ( here 's an example), and you will read that germination is poor in hot weather.
However I've never had a problem with germination or flavour in my lettuces sown all through June and July.
Even in a greenhouse I always get good germination with radicchio at the height of summer.
Though that is naturally a mediterranean plant of course.
gardening.stackexchange.com/users/99/Tea Drinker
For whatever reason my lettuce (except for the mustard) in my mesclun mix didn't take this year.  I'm about ready to rotate in something else, but I'd really like a decent crop of lettuce.   I don't see any reason why I shouldn't be able to plant lettuce and pick it while it's still really tiny right?  I thought the reason you shouldn't plant it in hot weather was because it bolts quick.  Does it just not taste good?
What happens if you plant lettuce in hot weather?
gardening.stackexchange.com
<vegetables><lettuce>
gardening.stackexchange.com/questions/502
gardening.stackexchange.com/users/10/Peter Turner
Lettuce likes cool weather and lots of moisture but there's no reason you couldn't grow it later in the year. It was generally believed that bolting was caused by higher temperatures, but this is not the only factor. It is said that cool weather is better to produce a sweet, tender lettuce crop. A reason for planting lettuce in cooler months is to give them a chance to reach maturity before they've produced chemical weapons against bugs.  
It was generally believed that bolting was caused by higher temperatures, but this is not the only factor. It is said that cool weather is better to produce a sweet, tender lettuce crop. A reason for planting lettuce in cooler months is to give them a chance to reach maturity before they've produced chemical weapons against bugs.  