I always thought it was because each time unit's size is a function of percentage of the total time experienced. In other words, a 2 month summer is a signific

I always thought it was because each time unit's size is a function of percentage of the total time experienced. In other words, a 2 month summer is a significantly larger percentage of the overall lifetime of a 5 year old as compared to the very small percentage those same 2 months represent for a 90 year old. Likewise in the formative years, you are constantly experiencing and learning new things. So the number of "aha" moments one would remember get fewer and farther between as one matures. By the time you're old and wise, you've seen just about everything and it takes more effort to have a memorable "aha" moment. Together, in youth you have a very large percentage of your life's time, along with a whole lot more novel experiences makes looking back at the time feel much longer than currently experienced.