First of thank you for doing your homework before bringing home a new snake. I don't see any problem with starting out with an older animal, in fact it might be better. You don't have to worry about getting tagged by a grumpy baby, and he'll have sufficient fat reserves if moving stresses him out a bit and he goes off food for a while. Besides, 5 isn't that old, he still has many years ahead of him. Good luck with him, I hope he works out for you!