Working on the assumption that some form of age gating for things like app stores and web sites is coming (the rights and wrongs of that are a separate discussion), I actually quite like this bill as probably the least harmful way to do it.
The bill itself doesn't say that the OS should validate the user's age, just that it should:
Provide an accessible interface at account setup that requires an account holder to indicate the birth date, age, or both, of the user of that device for the purpose of providing a signal regarding the user’s age bracket to applications available in a covered application store.
Similarly it must:
Send only the minimum amount of information necessary to comply with this title and shall not share the digital signal information with a third party for a purpose not required by this title.
So you set a flag (under 13, 13 to 16, 16 to 18, or 18 plus) when you create an account on your OS and that's all third parties see. They also can't request more information:
A developer that receives a signal pursuant to this title shall use that signal to comply with applicable law but shall not do either of the following:
(A) Request more information from an operating system provider or a covered application store than the minimum amount of information necessary to comply with this title.
(B) Share the signal with a third party for a purpose not required by this title.