I'm in my mid 30s now, and I started noticing grey hairs a few months ago. Like it haf to be under certain conditions of fluorescent lights to be visible. A month ago I realised, I'd probably look good turning gray, since it was more than like five individual gray hairs.
Funnily, this week I realised the grey is showing even under natural light. Although I always thought it'd be my beard going gray first, but that bugger doesn't seem to be affected as much.
And, as far as I can tell, I'm early about this in my family. Both my parents retained their hair color into their 70s, with my mom showing gray temples more than may dad.
A friend of mine got her first gray hairs when she was like 25, but that was stress related. Also, she's used to dyeing her hair anyway, so it doesn't matter much (but since hair natural color is more of a darker shade of brown, even single gray hairs may show really well).