I think it really depends on the 12 year old and how responsible and confident he is.

Ball Pythons make good starter snake IMO as long as you do the research first. They are docile and slow. Odds of you getting bit are low and if you let one escape out of your hands, something is wrong with your hands.

Corns and Kings are more forgiving on the husbandry is you screw something up. But they are not slow. They are quite fast and can take off on you. You have to pay attention to them while handling especially while sitting down. They are quite docile as well but more likely to bite than a BP from my experience.

So you have your pros and cons. In the end though it boils down to personal taste. The kid may prefer on over the other on looks alone.

Just my 2 cents.