tell age??

I bought my corn a few months ago, he was already sizable, now he is 39". I don't know his weight, he feels very toned and looks god, but I think he is a little thin. He is eating fuzzies, eyes just open size...one a week, should I feed him larger ones, or twice a week?

At 39" he should be on adult mice. Fuzzies are way too small for a snake of that size. And keep in mind that corns are slender snakes. A good test is to feel their spine, it shouldn't really obvious when you feel for it. I don't know what other species you've worked with, but compared to something like a ball python they will seem anorexic.