My open water class had several children of varying ages and backgrounds, so here are a few real-life examples:
Bio_girl: She was 15 or 16 and taking the class by herself. Noone in her family dived if I remember correctly, but they were supportive of her. She wanted to be a marine biologist when she started college, and was already a fish freak. She behaved well, acted mature, and aside from a few issues with mask clearing (like who didn't have those), she did good.
Monster and Son: This was a pair in my OW class, father and 10-11 yr old son. The son had trouble with tables and it looked like his dad did the homework. He never paid attention in class and often had to be corralled back to the rest of us. Of course the father wasn't any better and often berated his son in front of everyone, "What's wrong with you!? It's just a mask clear, get with it moron!" etc etc. I had just finished reading the book about the Rouse's tragedy (Last Dive), and I saw eerie similarities. Unfortunately they weren't in my check out dives so no idea if they passed or not.
Gnarly_dude: I got an ear infection after pool work and had to delay my OW checkouts. Because of that, noone from my original class was with me that day and I got buddied up with a stranger. He was a 17 yr old surfer dude, and seemed ok on land. In the water he was often playing around, but so was everyone else while we waited on the platform for the lost folks in the quarry to find their way to us. Aside from kicking my mask and regulator off twice (that could be my fault for being too close), he did ok and was a good buddy.
I think it all boils down to two things: Maturity and The Will to Dive. You need to have both to be a good learner and a safe buddy. Unfortunately it's sometimes hard to read them both in people, so you just have to do your best.
