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Abstract Objectives To examine the early effect of a world first national policy (Social Media Minimum Age Act 2024, which established a national minimum age of 16 years for holding accounts on designated social media platforms) on adolescent social media use, and to describe adolescent use of social media platforms subject to the Act, experience of age verification strategies, any efforts to circumvent them, and any perceived behavioural substitution or displacement.Design Observational study.Setting Community based study conducted across Australia.Participants Australian adolescents aged between 12 and <17 years at the time of implementation of the age restrictions.Intervention The Australian Government’s Online Safety Amendment (Social Media Minimum Age Act 2024), which requires designated social media platforms to implement reasonable measures to prevent users under 16 years of age from holding accounts.Main outcome measures Data were collected immediately before (baseline) and approximately three months after introduction of the Act. Co-primary outcomes were adolescents’ self-reported use of social media in the previous seven days (every day versus not every day) and time spent using social media per day. A sharp regression discontinuity design was used to evaluate the impact of the Act on social media use. Differences in outcomes at each side of the age threshold were estimated using local linear regression with triangular kernel function.Results Follow-up data were available from 408 of the 436 adolescents recruited at baseline. More than 85% of participants aged under 16 years reported using social media platforms subject to the Act at follow-up, predominately via use of their own accounts (54-68%), 66% of whom reported exposure to platform age verification, most commonly self-declared age (24-39%) or uploading of a picture (“selfie”) (13-27%). Efforts to circumvent restrictions, such as use of a “fake” account (15-19%) or social media access via a private browser (6-11%) were also reported. Between baseline and follow-up, daily social media use was stable among 12-13 year olds; reduced somewhat among those aged 14-15 years (from 78% to 69%), and increased for those aged >16 years (from 80% to 89%). Time spent per day using social media was relatively stable between baseline and follow-up for 12-13 year olds and those aged >16 years but was lower at follow-up for those aged 14-15 years (from 3.40 to 3.13 units on an ordinal scale). In regression discontinuity design analyses, insufficient evidence was available to support a discontinuity in social media use on these primary outcomes (P≥0.60).Conclusions Despite the intent of the Social Media Minimum Age Act 2024 to delay access to social media platforms and reduce the potential for online harms, little evidence was found of immediate substantive reductions in reported social media use by adolescents under 16 years.Trial registration Australian New Zealand Clinical Trials Registry ACTRN12625001056482.