I got my first gray hair when I was around 11 or 12. I have alot more gray hair now, but it's not noticeable unless you look, but I do have it.
My father had a full head of hair till the day he died, except for the year before he died when he tried to dye his hair and left the dye in all night, and as a result he had a couple bald patches on the back of his hair that grew back stone white.
On my mother's side, my grandfather was bald, and was going bald by the time he was in his mid 20s. And my uncle, on my mother's side, is also bald. He started going bald around his mid to late 20s. In his 30s, he was noticeably bald.
However, my brother and I have all our hair. My brother will be celebrating his 41st birthday next month, and he hasn't even suffered thining of the hair. I'm 26 and have alot of hair, with no indication of balding at all.