Maybe it's that whatever colour we start out with gradually fades from it's bright youthful hue to something duller and closer to grey?
My husband had golden blond hair as a little boy, but it was a lovely rich medium brown shade by the time he was about 8 or 9. (On a related note, I'm not sure why people call it "mousy". Medium brown is such a lovely hair colour.) difficult child had golden blond hair as a little boy -- little cherub curls too -- and he's still blond, though a shade darker. easy child and the Monsters all started out with jet black hair, and have lightened gradually with time to various shades of very-dark brown. Me, I've always had just-off jet black hair. It hasn't changed since birth, based on photos.
This is turning into such an interesting topic.