As one grows older units of time in relation to you life as a whole get smaller and smaller. When you are 10 one year is 10% of your life. By the time you are 50 it is down to 2% and continues to get smaller and smaller in terms of percentage with each passing year. That is one explanation I have heard why time seems to pass quicker and quicker as we age.
I hadn't heard it explained that way, xp, but makes sense. :)