All actors already know your age with 99% accuracy: tech giants (Google/Meta etc through your searches/choices) and government though your... birth. The same holds for your gender, job, spouse and more. And they share all of it to many other endpoints. They just want age verification to cover the rest 1% and to transfer the responsibility to the user.

The problem with any online age verification system is not only that many other info is leaked (personal, IP address etc) but even the fact that YOU apply for such a service and might use it for nasty things.