The gray is stress, Just look at everything he has to look after. Yuna, Tidus,Tidus which is a HANDFUL, Plus help defeat sin. I think he is aloud to look old.
Aswell do you think it may be that the makers thought that he is realy wise so to make him look like he is very experienced they made him look old?
i think and for an old guy he is still Hawt no matter wat colour hair!