As has already been said - completely normal. I found my first grey hair at 18, my Aunt was white by the time she was 28. I have been told that it is Irish genes that make your hair go grey/white early. My Great-great-Grandfather was Irish, so maybe it's from there.
Btw my cousin (son of Aunt who was white at 28) is also completely white, yet his father still has black hair (not a lot - but what there is is black) at the age of 86! And no, he doesn't dye it. A couple of years ago he was in hospital for 4 months and he didn't get any 'roots'!