It sets up the infrastructure for getting user ages and allowing services and websites to get an attestation from the operating system. Once that system is widely used and becomes ingrained, they can create a follow-up bill that demands the attestation be cryptographically verifiable by a trusted party.

In that scenario, the only way the operating system's promise that you're not a minor would be trusted is if it was signed by whoever holds the private keys—and that's definitely not going to be you, the device owner.

It would either be the government, or more likely, the operating system vendor. In the former case, now services can cryptographically prove that you're a resident of $state in $country, which is amazing for fingerprinting and terrible for anonymity. In the latter case, you can guarantee that only the corporations will be holding the key (like with Microsoft and secure boot), and you can kiss goodbye to your ability to access services on FOSS operating systems like Linux or custom Android ROMs.

This proposal is just a way to get their foot in the door with something palatable. If you've ever come across banking apps on Android using Google Play Services' SafetyNet feature to restrict access to only "secure" devices, you'll know exactly how this turns out: either you use the phone you own the "approved" way with a stock ROM where Google has more permissions than you do, or you're not doing your banking on your phone.