We have significant, long term, data that age restrictions reduced the use of alcohol and tobacco.
What has changed, in the modern era of internet age restrictions, is that the enforcement of age restrictions online has been reduced.
For example, online pornography has massive numbers of users under 18, but there has been little enforcement by law enforcement agencies.
The solution is quite simple. If a person under the age of 18 is in possession of an application that is prohibited for minors and the company that creates that application fails to verify their age, then the company should face criminal charges, just like we do with tobacco and alcohol.
To say that age restrictions do not work is to ignore almost a century of public policy.