How to tell a corn snake's age?

I just got my corn snake a week ago from PetSmart and he/she is doing really well. He's about a foot long and a centimeter thick, and the people at the pet store seemed to think he's around 6 months but that seems pretty old for how small he is. They also said he was eating two pinkies at a time but when I offered him a second one at the first feeding he didn't want it, and then four days later I offered it again and he ate it quickly. How old do you think he is and how often should I expect him to shed?