Do silver mitt ferrets gradually & naturally turn white as they age?

Ferrets can change coat colors at various times, including, like humans, going grey as they age. Every winter, two of my current ferrets get a beautiful, soft, mostly white coat. Then, in summer, they are back to their rougher, sable coats. Of course, this could be partly because their mommy had a silver-white coat, and their dad was a sable. But I have seen many ferrets slightly change color, and a couple that completely blew their coats and changed colors completely. It was quite odd.