How does that help Facebook? They already have plenty of signals to guess their users' age, what would they do with an other one? They are not going to ban children anyway.
Every one of these age assurance laws basically says:
1. The OS vendor must provide an age bucket using the minimum amount of data necessary
2. App vendors (i.e. Facebook) must use the OS vendor's age buckets to determine age
The idea is that the next time Facebook gets hit with a child endangerment lawsuit, they can say "Well, we used the age buckets the government told us to, and they said the plaintiff was 18+, so we're not liable".
This, of course, assumes that most social media and Internet regulation will continue being targeted at children only, both because courts are reluctant to enforce 1A on laws that censor children[0] and because the current political class actually benefits from the harms Facebook does to adults. Like, a good chunk of government surveillance is just buying data from Google and Facebook.
[0] The root password to the US constitution is "th1nk0fth3cHIldren!!1" after all
It helps them by making it somebody else's responsibility to get it right and thus shields them from liability.