eMarketer estimates that 82% of US teens ages 12 to 17 and 43.5% of children ages 3 to 11 will use the Internet on a monthly basis in 2009.

Comparative data from Nielsen Online indicates that about 19% of active Internet users in July 2008—or 32.4 million people—were under age 18.

Source: eMarketer
http://www.emarketer.com/Article.aspx?id=1006741