Might be a yearling that's been "maintenance fed", impossible to say though. His arrival @ Petsmart has nothing to do with his age, they buy from breeders & most breeders have some hold-backs or unsold snakes.
Yes, of COURSE he's stressed, as are all snakes in a new home. It's scary, they have no idea how or why their whole world they knew vanished & now they're some
other place. Best practice is not to handle any new snake until they've taken 3 meals without a hassle. They need time to settle in.
Why is his terrarium "bare"? That's also probably why he's hiding...should have 2 hides at least, one on cool side of cage & one on the warm side, + some "clutter" will help. Best if his cage is in a quiet location too. Can even cover part of the outside for his privacy, & keep kids & other pets away too, if you have them.