I have a double cousin (her dad is my dad's brother, her mom is my mom's sister) who was completely gray when she was 18 years old. She stayed gray for years because she never liked how her hair looked when she got it dyed. I saw her a few years ago (she's in her upper 40s), and she had the nicest looking blonde hair I'd ever seen! When I complimented her on it, she said she'd finally found a product that worked on her hair. I mentioned how nice she looked, and she just beamed!
I think gray hairs have something to do with genetics, like baldness (my brother lost all his hair except for a ring around the bottom half of his head when he was 18 years old; so the whole top of his head is bald).
I myself will turn 45 years old at the end of Jan; I noticed a few "gray" hairs in my hair starting last year. Now I'm waiting to see as I get more if I'll have silver hair or white hair! I see "the gray, white, or silver" as a badge of maturity. Hopefully, though, it will be quite a few years yet before I have a full head!