Isn't that question a little...uhm...strange? Places for under 18 year olds are, if they exist, made for exact that kind of residents those who are not adults. It would be strange to me, seeing an adult pretending to be one of them.
What you need to look for is somekind of roleplayplace....but I guess it can be difficult, because child avatars are a way more popular than teenagers. But they might be something to look for.