while the US one is at the OS level, it’s just asking you to provide an age that isn’t linked to your ID or anything.

It's Age Verification, which will almost certainly mean either ID scanning or facial scanning via the device camera. Or alternatively card transaction verification - the OG method baked into all these laws is the one that pays MasterCard and VISA. ID and facial recognition are cheaper services because the business providing the scan service can make more money off the ID or face they scan.