He dyed his hair.
Maybe. Do you have a source for that?
If not, see my #25.
I've met President Reagan on dozens of occasions, most recently in 2000. His hair is much more gray than photographs would indicate, though certainly dark for his age.
Some folks are lucky in that regard. My Dad is in his 70s, and is stiil more dark than gray. His brother two years older is completely white-haired.
