Isn't 21 like the optimal age to be? It is your point of lowest responsibility as an adult, peak physical condition so medical ailments are typically a non factor, old enough to do whatever you want. I still got a week until I'm 21, but getting any older than that sounds pretty damn depressing judging by all my older friends. Maybe it's because they are still in a small college town years after they should have left.
It's different for everyone. Most of the people you encounter in a small town, who stayed there from birth, are usually quite disillusioned by their mid-20s. That's the usual age when mortality subtly creeps into a person's mind. You're kind of a point of no return in terms of personal development. There is little schooling left to change a person's course, and even if there were plenty of options, people are already work on a rut by this age, so on a deep level, they're comfortable. A few kids, a wife and job usually finishes the job, and they become docile.