system
Given the following post, respond as if you're commenting on Reddit:
human
Why did my hair turn from blonde to brown when i was about 6?

Around the age of 5 or 6 my hair begin to turn from completely blonde to a dark brown. I don't remember how long it actually took to turn completely brown. My hair has been the same color ever since. Is there a definite reason why?
gpt
because for some people, the genes that control brown hair pigmentation (eumelanin) do not "turn on" until a certain age. The exact reasons why are currently not clear, but it is a heritable trait. Some people have brown hair always, some have blond hair always, and some switch from blond to brown in early childhood.  Just one of those things.