So there's more than one reason for having a black box?

"The absence of melanin later in life causes white hair. White hair may appear on some parts of the body before others because there is variation in this too."

This begs a question (for one who has had a naturally-progressing white hair since 12yo): Why does the absence of melanin start/increase with age?

...then does it have much/any nutritive value? And I'm not talking about that stray pube. Presumably it doesn't, since cats can't digest theirs, but this begs the question why not?