I've always thought that one's perception of the speed of time is directly proportional to how much life experience one can remember.

Look at it this way: when you're six years old, you can't remember much more than the last couple of years, if that. So one year of school is equivalent to half your (remembered) lifetime, which seems like forever. On the other hand, once you're twenty-six years old, two years is just a fraction of your remembered lifetime, so it seems to go by much more quickly.

It also depends on how much you're living your life, of course. When you're busy with an interesting career, a tour of Europe, or an active social life, time seems to go by a lot faster than if you're living alone on Social Security. But that is an independent phenomenon which even Albert Einstein commented on:

When you are courting a nice girl, an hour seems like a second. When you sit on a red-hot cinder, a second seems like an hour. That's relativity.