This is an interesting point: there is a trade-off between kids being denied access to inappropriate websites and adults not being forced to verify their age. We can't have both, so we must weigh which is more important. One could argue that protecting kids is clearly more important; on the other hand, there are way more adults in the world than kids, so more people are impacted with restrictions for adults.
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.