Laws have been written that mandate that applications and services treat various age ranges differently. You can't serve porn to kids, kids under 15/16 have very strict privacy regulations in some areas, and certain content age ratings are an industry standard only because that was the industry way of getting out of being regulated.

For kids this is a privacy risk because there are so many thresholds. Once you're above 18, all you need to care about is "are you a pensioner".

Kids have been lying about their age since the dawn of the internet and people have started catching on, so now we're getting actual restrictions rather than the assertion that nobody in their right mind would click "I am 18 or older" to access a website.

Californian law mandates that operating systems keep track of their users' age, so of course Android must follow. In all of the cases listed on Google's blog, though, the option to not share information and download an app that doesn't ask for your age range is still an option.