IMO, the approach of having the large / popular commercial OS platforms ask you the birthday of the primary user on install (and secure that so it can't be changed), and then reveal the age (bucketed to a range) to apps. If you don't have kids, or care what they see, just put Jan 1 1900 (or have an explicit opt-out, which puts you in the last bucket). After that it's up to parents to parent.
How can that be? The world population has been growing for decades.