The same way you prevented adults buying pornography for kids prior to the web, and the way you prevent adults from buying beer for kids now.

Namely, you don't prevent it (I was 11 when I first saw hardcore pornography, on a VHS tape, at a sleepover party), but it does place a (surmountable) barrier in the way, which will reduce access to some degree. The degree to which that happens depends on a lot of things that are hard to predict. We have culturally normalized access to a lot of things for children, and reversing that will likely take more than just changes to a law.

Sometimes this question comes up with an implied subtext of: "It needs to be bulletproof."

It really doesn't, and especially if the ostensible rationale is blocking the ills of social media. If your friends aren't there, there's less motive to waste a bunch of allowance-money dealing with a sketchy adult to get there.

If it's good enough for beer and cigarettes it's good enough for social media.

> presenting only ID to the cashier the same way as buying alcohol

Selling alcohol to minors is illegal in the UK. Some do circumvent this by various means (e.g. fake ID or having an adult purchase on their behalf, both of which are also illegal), but the same is already true for the current age verification system.

Same way you prevent selling beer to kids. Impose harsh penalties for violators.

How do you prevent kids verifying as adults?

That's the same question.

Meanwhile apparently 70% of Australian under-16's retrained/regained access to social media.

See, even intrusive, surveillance and privacy-busting methods don't work.