I've a friend who's 28 now and her hair is all grey and white. Dead sexy. She first found white hair on her head just before her 20th birthday... I thought it's actually not a big deal, external signs of ageing, as they do not necessarily betray... your age ;)
I am a lot more worried about the health-related things though. Like, I've been having lower back pains on a regular basis for the last two years, and all my aunts and uncles went like "well, you're good for it until the end now, and it worsens with age". Sweet Hangovers don't seem to get any better with time either. I can't drink half as much a I used to when I was 16. But maybe THAT is for the best...