It's very hard to even just guess age without the hatch date! Growth rings tend to be unreliable, and size can be too, but how big is he? Another thing you can look at is if you can tell his gender yet. (tail length, nail length, are they staggered?)
My first male is 3 and a half inches long and 3 and a quarter inches in width. The second is 3 and three-fourths in length and 3 and the same in width. Both have been verified as males by myself and other people on torts forum just to be sure. They both have thick tails, longer front nails and male counterparts. The rings are extremely hard to see on their shells and very close together.