Actually, it really doesn't matter much if you get a baby or an adult. Both make great pets.
True babies are a little more work, and can be easier to startle or upset, but how do you think all those older snakes got such good temperaments ?
If you don't mind a little bit more effort, go with a baby so you can watch it grow up. It's pretty rewarding.
If on the other hand you doubt your snake-charming abilities, go with an older snake that someone has already taken down the road to being a good pet.
Either way you go, I have to warn you....
You'll never stop at just one!
Gale