There is already a relatively large sample of exactly what teens do in SL. It's called the teen grid. For the most part, we make stuff and talk to eachother. Also a fairly large group of people form armies and wage war with the other armies, but in reality it's not at all intrusive. Though "You're flying! You have 0.1 seconds to stop before you get teleport home!" *teleport* is annoying.

Personally, I'd imagine something similar to the system which has already been in place for teen accounts on the MG for years to be used (why is that system in place? I have absolutely no idea.) We can't teleport into regions marked as "mature,", and we can't see into them - they appear as holes in the world. PG sims offer no issue whatsoever, when used correctly. And we've seen that Linden Lab have no issue with deeming a massive group's use to be "incorrect" and booting them out. This distinction would not be new.

Additionally, I feel that issues two and three are already resolved - the Teen Grid has fairly clear cut rules on the matter: no sex, no nakedness, no swearing, no real life information. Violence is apparently fine, though there's never been any discussion of extremely graphic violence (because nobody's ever tried it, that I know of.)

Intrusive age verification is entirely unnecessary (the MG currently has no verification, and the TG requires you to be over 18 (eh?) to register). But I wouldn't put it past Linden Lab to demand it anyway, given previous attempts at it.