Sure here's one example of decentralizing it -- it's going to be overly simple just as a toy example to show how easy it could be:

Whenever you want to prove your adult you go to "am I an adult.gov" and you use your credit card or whatever to prove you are an adult. At which point you get a 1-time 5-digit code that is UNIVERSAL TO EVERY SINGLE HUMAN and good for 1 hour (everybody who uses the site gets the same code that hour).

Then when you want to look at porn or something, you use this code. Boom simple and done.

There are even much better much more private techniques that use cryptography, and AI is happy to explain these graduate-degree level topics to you at your own pace.

Of course there are situations where people steal things, and use deep-fakes, etc, but those exist in every model.