How do we find out how old our new iguana is?

My Husband and I just got a green iguana. The question we have is how do we have an idea of how old it is? from his head to the base of his tail is 3 1/8 of an inch. from his head to the tip of his tail is 12 inches....anyone having an idea of how old he might be would be a great help to us for keeping track of his growth. Thanks!