NBC reports that Teenagers & Young adults are online longer than on Television per week.
In addition:
The American Academy of Paediatrics has published a report of possible negative health effects of television viewing on children and adolescents.
possible negative health effects of television viewing … such as violent or aggressive behavior, substance use, sexual activity, obesity, poor body image, and decreased school performance.
Update: The Washington Post reports on Teens are spending more time consuming media, on mobile devices