Not sure if this a birth mark exactly, but I have come across a family branch (KERR of Argyllshire, Scotland) where many of the descendants experience white/gray hair very early.
In one family, the mother & father were actually related to each other (1st cousins, once removed) so it looks like the chances of inheritting this gene were doubled - all 6 of their children went white early. Some of the grandchildren have done so too. And by early, I mean some were snow white by 19 years old!
Has anyone ever come across this before?