Sure, it would massively restrict access for most people. But what if you just would have to buy a device in a store to get access to a VPN to view "adult content", and you need to show an ID to buy it. (similarly, make SIM cards age-gated; they often already require ID).

It's easy (both in implementation and policy, and easily enforceable after a grace period), devastating in impact, and most critique of age-gating or whatever curr. considered wouldn't apply.