If anyone had doubts about rapid growth in the virtual-worlds sector of cyberspace, this should clear them up. There are now more than 150 virtual worlds – either open now or in development – targeting people 18 and under, up from around 100 just last April, according to Virtual Worlds Management (VWM). [The full list is at that link, though the definition of "virtual world" seems to be broad - I noticed one site that's largely avatar chat, not a whole "world."] "In all there are 95 youth worlds currently live. Another 68 are in concepting, development, or testing phases. Tweens' worlds (for ages 8-12) lead at 88 of the 150+, kids' (7 and under) come in second with 72, and teens' third with 60. Disney alone has nine in development, VWM reports. The New York Post cites eMarketer research showing that estimate that "more than half" of all online youth 3-17, or about 20 million young people "will visit virtual worlds by 2011, up from 34%, or 12 million, this year." Here's some analysis about the VWM report from its authors. I noted a comment in it about virtual worlds "aging with their users" from Craig Sherman, CEO of Gaia Online, a world targeting 13-to-18-year-olds. He told VWM that 30% of Gaia users were now 18+ and the site had, "accordingly, grown a little edgier" (inevitable, undoubtedly, but something for parents to be alert to, with kids and adults sharing an online community). It's logical that people wouldn't suddenly drop away from a site targeting youth just because they turned 18.
For a whole range of man-on-the-street views of virtual worlds, see this fun video from Global Kids in New York, or read coverage of a conference in youth learning in virtual worlds last fall from CNET. See also my recent item on ways kids have found to game the system in virtual worlds, sometimes for the purposes of cyberbullying.