As I've grown older, my hair color has changed noticeably. When I was a little kid, my hair was very blonde, however as I've grown older, my hair color has changed to a much more brown-ish color. It doesn't bother me, its just something I've always been curious about. Is it genetic or some external factor that causes it?

Hi,
Hair color is associated with Melanin, a body chemical. Here is a link:
http://en.wikipedia.org/wiki/Hair_color

Usually, this is genetic, or inherited from somewhere along the family lines.
My wife's hair, at early age, was reddish. When she was 18, it was black, naturally.
My own hair, at 33, started changing to a grayish color, then at 50, almost back to dark brown.
The website I gave is interesting. I wish you the best.