I already knew that Grindelwald had pale blond hair -- and it doesn't seem much of a step to bleach it white.
Picture of Albus Dumbledore and Gellert Grindelwald
As a young man, Grindelwald had golden blond hair and a "merry, wild" face.
Picture of Grindelwald as he is stealing the Elder Wand from Mykew Gregorovitch.
I've seen too many folks around here with obviously dyed pale blond and/or pure white hair, who are apparently in their teens, to give much attention to hair colour as an indicator of age anymore! (I also saw one person with purple and blue hair, with a couple of streaks of green, which was awesome and great.) As for faces, well, one reaps what one sows? Grindelwald has not been a very nice person at all ...
Another picture of Albus Dumbledore of that time period.