I guess it's just a genetic thing -- my grandmother, from whom my mom got her black hair, also didn't go gray until very late in life -- at her death at age 95, she still had a few dark strands. My dad's side, filled with blondes and redheads, all went gray very young -- my dad was blonde, and by the time he was 50, his hair was entirely white. His sister is a redhead, and I don't know for sure, but I suppose the reason why she's still redhaired at nearly 70 is because hers is also entirely white and she just covers it up. 
Andria
Andria