There are a number of psychologists that agree with your son with one refinement -- the reason time speeds up is because you're spending more time dealing with familiar situations. You don't learn from them, you just deal with them so you don't bother to remember them. When you're young, everything is a new situation, so you are more aware of each passing moment since you're learning. The corollary is that if you want to have a long (and happy) life, keep learning.
