First noticed a couple of definitely white hairs almost as soon as I turned 30, but they were only a few months old so I really can't be sure if I had any before 30 or not. Now at 33 I still only have a sprinkling (about 10 or so) that I can find when I look for them. I have a fair few translucent hairs from close to my hairline (long though, not baby hairs), but I don't know if they count as depigmented or if those hairs have always grown like that.