If someone's hair is streaked with gray, you can describe it as grizzled. Your dad's grizzled beard might need a trim by the end of your two-week camping trip.
As well as a good way to talk about the silvery hair (or facial hair) itself, you can also describe a person as grizzled. Before he went totally white-haired, Santa Claus must have been grizzled. This adjective comes from a 14th-century noun, grizzle, which meant both "gray-haired old man" and "gray horse," and probably came from the Old French grisel, "gray."