also for the record, my husband married me when he was 21, and I was 22-and there were no purity rings involved! :-P
We were together all through university, and the wedding was after our graduation. We were young, but it is not as if we did it out of high school or something. I don't think it's too young to get married when you're graduating from college.
I think the Jonas Brothers seem even younger than they are because they work for Disney and their fans are typically preteens. I don't think he's making a bad decision based on his age alone, but I will agree those with purity rings often do get married young in general.