I'll be honest, this is something that just doesn't bother me. Maybe it comes from having co-ed bathrooms at college, I don't know. I don't have boys, only girls, but I'm just not bothered by this. I don't feel unsafe, I don't feel uncomfortable, and I don't feel upset.
I'm not all that concerned with stating an age cutoff (as you can't tell a child's age just from looking), but i am concerned with actions. As long as the person is behaving (not peering into the stalls, trying to climb under them, not harassing others), then I'm just not personally fussed about it. If they are standing and waiting or using the facilities, then whatever, IMHO.