It's mostly a matter of opinion, and like most of you guys have said, how mature they are. But you wouldn't see a 9 year old kid acting like a 16 year old. It also depends on what you allow and what type of server you have is. If you allow swearing then I don't think they should even be on the server, not because they'd ruin a game, but because they shouldn't be hearing that. And if you a run a major server with a bunch of complicated plugins, then the staff should at least know how to use them. But the main point is how mature they are.
