PARIS, Aug. 17 -- For every French teenager who spent the past year watching Parliament debate whether to lock them off TikTok, Instagram, and Snapchat until they turned 15, the answer arrived on Thursday from the country's highest legal body: not yet, and not like this.

France's Constitutional Council struck down legislation that would have made France the first European country to ban social media access for anyone under 15. The ruling did not dispute that governments have an interest in protecting children online. What it found, instead, was that the bill as written had never solved the problem that sits at the centre of any such ban.

To enforce a minimum age on a platform, the platform must verify the age of everyone using it, inclu...