Centralized platforms, including the big federated ones, might be required to institute this. We'll have to wait and see. Usually laws define the words they use. So they'll have to define "social media" and "age verification" and what does and does not apply. After it's out, then it's up to the courts.
I've been watching the Section 230 discussions in the US, and they don't want to wipe away what's there because they want to protect smaller platforms, but they are looking to amend it to add additional wording for size, revenue, etc.
Also, there's NOTHING stopping you from running your own service and hopping on. Honestly this is why I want to see self hosted plug and play devices for the Fediverse. You don't need age verification if you run it for your family and know everyone's age.
These age verification laws will work on some folks, but I think the vast majority of folks will just be pushed to the outskirts. And honestly, we're it. This could be a boon for the Fediverse.