[Coda]

If I had to guess:

When melanin breaks down, it loses its pigment. Living cells will regenerate the melanin when they get damaged (which is why people tan in the sun) but hair isn't made of living cells.

The longer your hair is, the more time the hair has to get damaged -- sunlight, shampoo, sweat, whatever. The oldest part of any given strand of hair is the tip.

So if you have the kind of genetics that would make your hair predisposed towards going gray, then your hair might do that even if you're still producing dark hair.