Laws and lawmakers just concern themselves with making broad "laws" with little regard to specificity and applicability. California, Colorado and Illinois mandate OS "providers" to generate a signal. It is a copy pasted bill with little grounding in reality but a lawmaker is not going to say no "protecting children".
Pushed by AVPA - a group of companies standing to profit from this: LexisNexis, some Thiel corp, etc.
California's law explicitly requires the system and apps to take the user's word for it and not use other information to determine age, which more and more feels to me like kind of a brilliant move to cut the legs out from under other attempts to use the same for surveillance while still satisfying all the surface-level "protect children" sound bites.
You missed US states competing on setting up age verification legislation that lets anyone sue any developer who produces systems that don't do age verification for life-destroying amounts of money.
Eh private prosecutions and third party standing are generally disfavored to such an extent that sure, attention-whoring legislators will propose it, but whether it even passes constitutional muster on the state level is an open question, and open in every state.
For what it's worth, the "verification" in the California law (not a bill, it's already passed and takes effect 2027) is basically the Steam birthdate popup interstitial. There's explicitly no actual link to any outside information, just requiring that the system save a value the user sets and then that apps use that value for any age gating.