2 year old cat has graying hair?

I have a neutered indoor tuxedo cat, breed unknown, although his vet believes he may have a bit of Maine Coon in him. Other than that I just consider him a Domestic Longhair. He will be 2 in September of this year. He is healthy, aside from a little extra weight that we are working on. However, I've noticed lately that there are several places where his black fur is turning gray, such as behind his ears and on the backs of his legs. I know that animals gray just as humans do. But he seems much too young for that. I read that stress can cause it, but I can't imagine he has too much stress in his life. I've looked closely, and his skin seems healthy underneath. What could be causing his fur to change from black to gray at such a young age?