Make a law where if you ever sell a minor’s data, regardless of whether that minor lied about their age or agreed to a contract (wouldn’t be binding!), then you face steep fines. If you’re found buying data belonging to a minor, also steep fines. If you buy and then de-anonymize data belonging to a minor, really steep fines. To collect and hold data that may belong to a minor, you need to justify it (like GDPR).
Obviously this doesn’t scale for aggregators, advertisers, basically SV, which is the point.
A restaurant is responsible if they sell booze to a minor. A store is responsible if they sell cigarettes to a minor. Serving a minor that lies about their age does not get you off the hook!
Problem solved. User data is toxic. You won’t want to hold onto it for any other reason than a legitimate use.