I had a few white hairs from time to time in my early 20s. Now I'm 29 and getting perhaps a few percent grey/white hairs on both sides. But men on both sides of my family have gone full-on grey/white from 50 or so so I'm not concerned (I quite like the idea really).

One of the signs of serious stress is going bald or having clumps of hair falling out. If you're not at that level, I wouldn't pay much attention to it. But, as always, see a doctor if you're worried.