I'd argue that the reduction of underage smoking has much more to do with things like social acceptability and education about the dangers of smoking, and not about physical controls on the distribution of and access to cigarettes. There also appears to be a recent trend of younger people not drinking alcohol to the extent that my generation and Boomers did, which is wonderful, but probably has nothing to do with physical access to beer.

This is the right way to reduce childhood social media use: Make it socially disgusting, and make it widely known to be dangerous.