My hairs started to grow white when I was about 18. After that, I notice some of them grow black again, but mostly they are turning into white.
At this age (24 y/o) almost half of my hairs are white. I've visited 3 doctors (different ages) but they didn't tell me anything other than it's genetic. However, there is no one in our family having such a condition.
I've asked my parent and grandparents to recall sharply if anyone related to them had her/his hair grown white in a young age, but the answer was negative.
This doesn't end here. My beard and mustache are very sparse. If you split the hair coverage on a normal male, then I only have the bottom half. Even those don't fully grow.
Again, no one else has such condition in our family.
Could it have another reason other than genetics? Since I don't live in some radioactive field or some exotic locations with strange environment.