Club Penguin is one of the fastest growing virtual worlds for children. As of August 2007, its registered users exceeded 12 million and paid subscribers were over 700,000, according to Nielsen//NetRatings.

"Of course, virtual worlds are not new, but the level of development activity, venture capital investment and consumer interest in virtual worlds is unprecedented," Williamson told eMarketer.

eMarketer estimates 41.5 percent of children between three to 11 will use the Internet at least once a month in 2007, which represents 14.9 million children online, rising to an estimated 16.6 million in 2011.

Among teens, the Internet is even more prevalent. eMarketer estimates 76.4 percent will go online at least once a month in 2007, rising to 87.1 percent by 2011.

Overall, children and teens make up 18.2 percent of all U.S. online users.

"eMarketer estimates that 24 percent of the 34.3 million child and teen online users in the U.S. will use virtual worlds at least once a month this year," Williamson said. "And by 2011, 53 percent of them will be going virtual."

As more children and teens start to use virtual worlds, their viewpoint on the Web changes, too.

"They are growing up not only with social networking but also with the ability to interact with people, shop, learn and play in a graphic environment," Williamson added. "Flat Web pages with clickable links and banner ads may pale in comparison."

eMarketer expects that virtual worlds — particularly those for kids and teens — will see an increased level of interest from marketers in a next couple years.

"The intense activity in virtual worlds for kids and teens is only a microcosm of the larger development work being done in virtual worlds," Williamson is quoted by eMarketer as saying.