As an occasional TD myself, a parent of an 13 year old who's not even a Rec or Adv level player, and someone who's deeply involved in trying to encourage kids to participate in disc golf, I've gotta say that there's a lot of sides to this issue, most of which have been mentioned already. As a parent, I've tried to make sure that my son is aware of the rules and course etiquette (given that he's borderline Aspergers, it usually doesn't take much more than letting him know what the rules and etiquette are -- at that point he'll do anything possible to avoid violating them). I also make it a point to contact TDs of any event (sanctioned or not) that he's considering playing beforehand to determine what level of participation from juniors they're anticipating, and what their overall tone about it is -- I don't want to put either them or my son into an impossible situation. I'll also contact any other parents of juniors I know of who might be considering the same event, to gauge whether we can collectively guarantee three or four kids so that there's enough for at least one card.
In my experience, though, most kids who're playing a sanctioned event aren't going to cause too many problems for other players from a speed-of-play or distracting-behavior standpoint. I worry as much or more about the effect just the presence of a kid on the card will have on the mental aspect of the game for other players -- if they feel like they're having to constantly watch their language, behavior, etc., that in itself can be a distraction, and can put someone off their game or reduce their enjoyment of the event simply from having one more thing to think about. In my son's case, as a thirteen-year-old boy I doubt there's anything he's going to see or hear on the disc golf course that he hasn't seen or heard before, so I don't give it much thought myself, but I know that it has to cross the minds of other players.
I also try to judge carefully whether to arrange things with the TD so that we end up on the same card if I'm playing. It certainly makes it harder for me to focus on playing my best if I'm constantly monitoring his behavior and play, but I also feel an obligation to make sure that if he's playing in an adult group, particularly on a course he doesn't know, that (a) he's got someone he knows and trusts on the same card and that (b) there's at least one fewer adult player on the card who might be distracted or bothered by him, since I'm taking up that spot.
And as has been pointed out already, there's lots of kids out there who absolutely play at a level that justifies them playing Intermediate or even Advanced. I got to see a lot of them caddying for my son at Am/Junior Worlds. My son's new buddy Nick Duran just took 5th in Advanced at a tournament in Texas this weekend, just eight strokes back of the winner in a three-round event (he has three world championships in the 10 and Under division, and took third in 13 and under this year, at age 11). This year's 13 and under runner-up, Tristan Covington, has only played a handful of sanctioned events, but when he has it's been Int or Adv, and has cashed each time. I can promise you from having watched these kids that if you end up on a card with them, having their behavior be a distraction or having them slow you down is the last thing you'd ever have to worry about. They can school most adult players not only when it comes to throwing, but when it comes to conducting themselves appropriately on the course as well.
When you're down at the lower end of the Rec or Intermediate (if Rec isn't offered), you're going to have a certain number of people who're no fun to play with, who don't know how to behave, who play slow, and who cause problems for others, regardless of how old they are. Just goes with the territory, like having a slow time in previous races gets you seeded way back in traffic in a big running event like the Peachtree Road Race, which turn makes it harder to perform your best.