Your perception of time is based entirely on your memory. My guess is that as you get older there is an ever-increasing total of weight on your memory, so it tends to leave more blank spots in between experienced seconds or minutes out of the day.
But I can slow down time if I want. All I have to do is meditate and focus on almost everything being experienced in the very present moment. The sheer amount of information being recorded in my memory as I do that seems to expand my remembered perception of time, in order to encompass all of the senses I have placed my attention upon in each moment.
To support that, it's commonly reported that psychedelics have a way of slowing down time in one's mind. They also generally cause you to experience your senses more vividly in the exact moment that the senses occur.
In one sentence, I really think that time, as we perceive it in our minds, is a flexible variable that depends on how much information is being recorded to memory at the moment.
In fact, I must make a graph to show how this works psychologically. I figured it out a couple weeks ago.
Edit:
http://img340.imageshack.us/img340/7936/timegraph1200.jpg
That's my theory.