It seems unlikely that "is user adult" is not already easily modeled by any of these companies to within a very high degree of confidence. Even 15 or 20 years ago Google search could bracket your age pretty effectively. It doesn't seem like this adds metadata that wasn't already there.

Google prompts me to verify my age on my account I created in 2004. They’re not trying too hard.

If they admit this, they wouldn't be able to advertise to children anymore without breaking many rules.

Except that in the legal sense, "is user adult" flips from false to true overnight, and there isn't an easy way to account for that in any model that doesn't include verified ID. Same reason many liquor stores ID anyone who looks younger than 40.