I've thought about this often, in recent years. I'm 31 now, and I started noticing the apparent acceleration of time in my 20's. Particularly how fast Christmas seems to come. Do you remember when you were in grade school (high schoolers, this doesn't apply to you, as grade school was just a few years ago), Christmas seemed to never get here. Now, it seems like it's here before you know it. I saw Halloween stuff at Walgreen's today, and started thinking how fast autumn came.
I think the poster above was correct, in that it has to do with the fraction of your life that a year makes up, gets smaller. When you were 5, Christmases (or any annual event) seemed to be so far apart because they were 1/5 of your life. At 31, a year doesn't seem so long.
Also, I've noticed how fast 5 years goes by. I just celebrated by 7th year at my company, and it doesn't feel as long as the 4 years of high school was. I can't believe that since I started in 1999, kids who were starting 6th grade have completed that year, completed Jr. High, and completed High School. When I was graduating high school, memories of my 6th grade year seemed way more distant than memories of my first day on the job do now, even though they represent the same number of years.