My husband found this "snapping turtle" and wanted to keep it as a pet.

I took a look at him and his shell isn't right and he's rather docile. After searching on the internet, I would guess that he's a mud turtle, but I can't say with any certainty which species he is.

I live in East Texas. This turtle's shell is approximately 2.5-3 inches in length.

If this turtle is a mud turtle or some other semi-aquatic turtle, would it be possible to determine his age from his size? Also, can these turtles be successfully kept in captivity? My husband would really like to keep him.