A third of children aged between eight and 17 with a social media profile have lied about their date of birth so they appear to be over 18.
That is the finding of research commissioned by regulator Ofcom amid calls for more age verification tools to be places on internet platforms to protect children from harmful content.
Some 77% of children in the surveyed age range have at least one social media profile, with 60% of those under 12 having one despite most sites having a minimum age requirement of 13.
The study estimated that about 47% of children in this group set their age at 16 or over, while 32% had set it at 18 or over.
Within that group, the study estimated that about 23% of those between eight and 12 had a profile with an age set at 18, exposing them to material aimed at adults.
It comes as the government is expected to bring the Online Safety Bill back to parliament.
The bill would compel platforms to protect users, particularly children, from illegal, dangerous and harmful content, with large fines and possible blocks on sites that breach the rules.
Some platforms, such as pornographic sites, may also have to use age verification methods to prevent children from accessing content.
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Mark Bunting, director for online policy at Ofcom, said: “The protection of children sits at the core of the new online safety laws, so as we prepare for our new responsibilities we’re continuing to build our evidence of children’s lived online experiences.
“Today’s research explores the triggers that may lead children to experience harm online, including the risks of signing up to a social media platform with a false older age. This can put them at greater risk of seeing potentially harmful, age-inappropriate content.
“The children and parents in the study spoke of the potential tensions between online safety concerns and the need for greater protection, with the desire for youngsters to feel socially included and to have freedom to learn how to manage risks through experience.”
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