Based on a group of 24 children, 42% said they had registered as aged 18 and over, while 83% of the kids had lied about their age to register with a social network.
The ASA examined ads served to children on social networks including Facebook, YouTube, Stardoll and Twitter as part of resarch aimed at finding out what ads they engage with online, when it unearthed that kids were lying about their ages.
Of the 218 ads served to those who had lied about their age, 11% were for products that should not be targeted at under 18s. Of the 427 ads seen by the entire sample, 98% adhered to advertising rules.
The ASA said it will talk to advertisers about "self-declared age-gating", adding it would also raise the issue with social media companies.
It said: "Our report clearly asks questions of social media owners around the effectiveness of age-verification and whether enough is being done to prevent children from accessing age-restricted content on social media sites. We will be raising these issues with social media companies."
The ASA added that the research showed that advertisers "are working in good faith by taking into account the registered age of social media account holders".
It will present the research findings to its council and raising the issue with the Advertising Code writing body, the Committee of Advertising Practice and would assess whether new guidance was required for targeted online ads.
Guy Parker, the ASA’s chief executive, said: "On the face of it, our survey suggests that advertisers are sticking to the rules but children aren’t. But before we all lay the blame with parents and guardians, we need to be honest: if advertisers and social media companies know that children say they’re older than they are, don’t they have a crucial part to play too?
"We’ll be talking to them about self-declared age-gating and considering whether we need to take a tougher line. But we all need to be part of this conversation about how best to set the boundaries within which our children explore the world around them."