Hundreds of thousands of children aged between 8 and 12 are using Facebook, Bebo and other social networking sites in clear breach of their age-restriction policies.

Facebook and Bebo have a minimum age requirement of 13 and MySpace of 14, but a study by an internet security company suggests that the sites are unable to enforce their own rules.

Almost a quarter of children aged between 8 and 12 say that they are regular users of one of the big three sites, which suggests that they could have as many as 750,000 underage users between them.

Experts say that parents are not to blame if they are unaware of the age limits. The sites do not promote the age restrictions sufficiently and could tackle underage use easily, they say.

Age restrictions are not enshrined in law, but are part of the self-regulation that largely governs the industry. The Government is considering tighter legal restrictions if it concludes that the industry is not properly protecting young people.

Tom Ilube, chief executive of Garlik, which conducted the research, said that the social networking sites could do more to enforce their own rules.

“Facebook, MySpace and Bebo need to take their age restriction policies far more seriously. They know they need to be on the site somewhere, but do not want to stop younger people signing up, so they generally put them into the terms and conditions,” he said. “If a warning flashed up on the homepage stating clearly that it can only be used by those aged 13 and over, you would see an immediate change.”

Facebook asks for a date of birth during its registration process and rejects applications from anyone who admits to being under 13. The age restriction itself is in the terms and conditions.

Bebo declined to comment on the research and accusations that it was not taking its own rules seriously.

A spokesman for Facebook rejected the criticism and said that it had a process for checking the ages of users by contacting their online friends.

“Facebook leads the industry in designing for safety through our privacy controls, which includes the prevention of under-13 sign-up and stringent default settings for 13 to 18-year-olds,” the spokesman said.

Gillian McArthur, 48, was shocked when she found that Bebo and Facebook had age restrictions. Her son Oliver, 13, had been on Bebo for three years and on Facebook a year by the time she found out that he should not have been on either.

“I was astonished and slightly annoyed. How could Oliver and all his friends have been on there all this time and it is not being policed?” she said.

As a user of Facebook herself, Ms McArthur, an architect from Norwich, said she was concerned that she had not noticed the age restriction when she joined. “It is very much in the small print. I ticked the terms and conditions box but like most other people I didn't go through them.”

She said if she had found out before Oliver had turned 13 she would have been in a dilemma and would not necessarily have ordered him to take down his profile.

“All his friends are on there so I would not have immediately ordered him to get off. These are the moral judgments parents are having to make these days. It might make life easier if the sites were policed more robustly,” she said.

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