Something that I didn't know until today was that Second Life is for adults. It's got a Mature Content rating on it. Like I just said in a PM to dyza, it's like the rest of the Internet. There's tame stuff, racy stuff and beyond racy stuff. Boomer Island is harmless from what I can tell so far.
You have to be 18+ to have an account. For those of you folks who are looking to have your kids show you how to do it, that's up to your best judgment as a parent.