I too went grey early dont know why just bad luck i suppose very annoying though, i started gettitng greys at 14 and was permantly colouring my hair since 24 i am now 42 so you can imagine it just becomes part of your life it you dont want it to be grey dont worry most of your friends wont be that far behind you it seems to me more and more people are going greyer earlier maybe its all the modern day stress, good luck.
Gilbert sydrome can be associated with poliosis ie loss of hair pigment in streaks.However having grey hair may have nothing to do,and just normal greying.