If our perception of events are in relation to our past experiences, that could also explain this phenomenon. For example let's say you think back to that carefree, fun summer when you were five years old and marvel at how long that seemed to last, compared to the summer at age thirty that you barely remember. Well that 3-month summer made up 3 / (5 x 12) or 5% of your life at age five but only 3 / ( 30 x 12) or 0.8% of your life at age thirty, so the memory from age five would be 6x more salient (ignoring other factors like uniqueness of experience, how "present" you are, visual information processing, etc). Or maybe you were just on a real bender that summer you turned 30.

edit: just realized emreb has a similar comment after reading further down the thread!