I've said before (somewhere, to someone) that I'm often hip to the latest trends simply because I work with about fifty boys and women aged 16 to 20something. When ginormous-bags-wherein-you-cannot-find-anything came back in fashion, I knew because doing bag checks at end of shift required a flashlight and excavation tools. I also knew the moment big earrings, nail polish, and skinny trousers were back in fashion.
When you have the occasional fashion-forward coworker, then you know when something like gray hair becomes trendy, even before your brother sends you an article about celebrities who've dyed their hair gray.
I hesitate to call it a trend, because I doubt it's going to break wide beyond the Kelly Osbournes, Gagas, and Kelis of this world, who are fashion-bold. But the trend was apparently started by model Kristen McMenamy, whose 45 and let her natural gray come through several years ago.
Some comment that this look (old hair on young women) isn't pretty, but looking pretty really isn't the point of an edgy hairdo. It's Dali, not Monet.