This statistic comes from here -- https://www.gov.uk/government/publications/parental-support-... -- a preliminary analysis of the consultation. The headline statement is:

    "Of the parents and carers of children aged 21 and under who responded to Question 12 on the full-length version of the consultation, 89% supported “a legal requirement for social media services to have a minimum age of access”."

However, what the government (and the media) are _NOT_ reporting is that the consultation also paid an independent firm to undertake a nationally representative survey of adults in the general population. The above document acknowledges this itself, by stating:

    Caveats and limitations

    Users should note the following when interpreting these results: 
    Self-selecting sample

    The consultation was open to anyone who chose to respond. The results reflect the views of parents and carers who were motivated to take part, and are not representative of parents and carers nationally. As with any open public consultation, respondents may differ systematically from the wider population in their views and characteristics. 
    Question routing

    These questions were only presented to respondents who wanted to respond to Chapter 2: Interventions for safer, more positive experiences. All questions in this section were optional. Finally, Question 13 was only presented to respondents who answered “Yes” to Question 12 (i.e. those who supported a legal requirement for a minimum age of access in principle). The 96% figure therefore relates to the level of agreement with a minimum age of at least 16 among those parents and carers who opted to respond to this Chapter and already supported some form of minimum age requirement. It does not represent the views of all consultation respondents, nor all parents and carers who responded.
    Full consultation only

    The figures relate only to the full-length version of the consultation, not the streamlined parents’ and children’s consultations.

Status of results

   These figures should be treated as provisional. A comprehensive analysis of all consultation responses will be published separately.consultation, respondents may differ systematically from the wider population in their views and characteristic

So, it's 90% of 9499 parents who specifically went out of their way to respond to a consultation widely heralded as being predetermined and about blocking access to social media. For context, in the 2021 census (massively disrupted by covid) there were 11.5 million schoolchildren and full-time students whose parents were the target of the survey.

The representative study isn't published yet. The provisional headline 90% number is.

[1] https://www.ons.gov.uk/peoplepopulationandcommunity/educatio...

Age verification tech could put children at greater risk, says think tank