I used to think it was increased responsibilities, ie time's up before its all done.

Then I thought - decreasing energy ...ie insufficient oomph...perspective.. time is moving fast in relation to what I get done.

Then I thought - increasing volume of worry and mental perplexity..ie insufficient focus, makes time disappear while one is not paying attention to the moment.

Then I thought it was neural network slowdown ...again makes time appear to be moving faster.

Today, I think its because we don't sit in a classroom every day and watch the minute hand, and because we don't have parents who make us stay home and be bored.