My sixth grade teacher’s hair turned white as snow overnight in Vietnam. He was in his twenties.
VN tended to do that to many. My hair started to turn gray in my early thirties, nothing to do with VN I would hope, just stated to turn gray. By the time I was fifty, all my hair was gray including my stash. ;-)
Back in high school, one of my classmates had a condition which whitened his hair. We were 17 and he had white hair on his sides.
We called him daddy groovy, and he was everyone’s best friend because he was always the dude who went to the liquor store for us.