Young teenagers use up the equivalent of an average working week in front of a screen.
And for the first time they are spending as many hours on the internet as they do watching TV, according to research.
Children aged 12 to 15 now dedicate around 17 hours a week to each activity, communications regulator Ofcom revealed, up from about 15 hours online last year.
The research also revealed that over a third (37 per cent) of three to four-year-olds are going online, by using a desktop PC, laptop or netbook.
After the watershed, between 9pm and 10pm, 14 per cent of four to nine-year-olds watch TV on their own.
Almost half (43 per cent) of five to 15-year-olds with access to the internet at home have a social networking profile, rising to 80 per cent of 12 to 15-year-olds.
Children aged between eight and 11 have an average of 92 friends on social networking sites, rising to 286 for 12 to 15-year-olds.
The study also found children are sending more text messages than ever before.
Those aged 12 to 15 send 193 texts every week, up from 91 each week a year ago, compared to the UK average of 50 texts a week.
Eight to 11-year-olds send 41 texts each week, compared to 23 a week sent last year.
Around 9 per cent of 12 to 15-year-olds who use the internet say they have been bullied online in the past year, with girls more likely than boys to say they have been targeted.
Meanwhile, the annual report found parents' concerns about content online and on TV are decreasing.
Children and Parents: Media Use And Attitudes found 10 per cent of parents did not have parental controls on computers, either because they do not know how to use them or they are not aware of them.
Ofcom's consumer group director Claudio Pollack said: "Children are not just using more media, they are also adopting some forms at a very young age.
"This highlights the challenge that some parents face in keeping up with their children when it comes to technology and in understanding what they can do to protect children."
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