The Secret Of The Hair Colour Of Derek

It turns out that Eric William Dane (Mark Sloan) is two years older than Justin W. Chambers (Alex Karev). Surprisingly, isn't it? Dane was born in 1972 but Chambers - in year 1970. Probably Mark's Sloan's hoary hair is the reason what maks look him older as he is. How has Derek Shepherd (Patrick Dempsy) managed to save his dark black hair being four years older than his friend and "wife thief" Dane (born in 1966) Is the answer genetics? Or does he colour his hair? How do you think?