Two reasons:
1) when you're a kid everything is new. As you get older you've experienced more things and life becomes a routine. Unless you do new things on a regular basis the years start to blend together.
2) as you get older, a year becomes a smaller fraction of the time youve already been alive. When you're 10, a year is 10% of the time you've been alive. When you're 40, a year is 2.5% of your existence.
