In my experience (76 years), the "speed" of time, as it is felt, is related to how much time has elapsed since birth (or early awareness). So, comparing time at 10 versus time at 50, one year is a full 10% versus 2%. Thus, a year passes 5 times more quickly at 50 (and 10 times at 100).

I would speculate, that time is a perception of accumulated knowledge. There is so much to learn when you're young; much less when your old.