My personal hypothesis is time is perceived to move more quickly the older you get because an hour, a day, a minute become smaller fractions of the time you've been alive.

EX: When you're 10, a year is 1/10th of your life, but when you're 60 it's only 1/60th. The magnitude of an additional year of life at 60 does not feel as great as it does when you're 10.