Teen virtual worlds have done away with penguin avatars, restrictive chats and overly cute animation and games. They tend to capitalize on the social aspects of virtual worlds and allow for more freedom of expression in avatars (your online persona). The downside is that you'll find more inappropriate language and behavior, although the best sites are still somewhat moderated and definitely filtered. Expect more outside advertising and the availability of premium goods and services that cost real-world money.
Teen Second Life is a pg-rated version of the other Second Life website. Open only to teens 13-17, it is designed to provide a more appropriate environment for teens to interact. There are staff people present while the site is open to help keep an eye on things. The 3-D virtual world is free to join, although a premium membership is available that allows you to purchase land. Much like the adult version of the site, Teen Second Life has some powerful tools that allow you to design your own objects in world.
Runescape is actually a MMORPG (Massively Multiplayer Online Role-Playing Game), but it doubles as a favorite hangout for a lot of teens. Choose a character, study up on building weapons, fighting and earning gold, and then set out on an adventure. I managed to get myself killed within an hour or two of starting, so just keep your eye out! Runescape is free to play, but a premium membership is available. Premium members are advertising free and have access to more games, land and features.
Aimed at teens, Habbo Hotel has a diverse, global user base. The premise is that you can build your own room in a virtual hotel. Graphics are chunky and pixelated, but have an old-school computing appeal. Habbo offers premium shopping (i.e. real money) for special items. It has some notoriety for a landmark court case involving the arrest of a Dutch teen for the theft of virtual furniture that cost real money.
There is open to everyone over the age of 13 and language and behavior is expected to be appropriate for even the youngest members. There has a number of fun games and activities that appeal to a wide range of people. Basic membership is free and a premium membership is available for a one-time fee of $9.95. Much like Second Life, There members can create clothing and objects for their own use or to sell in-game. The currency is "Therebucks" which can be earned in-game or purchased with real world money.