The simplest solution is to just require ID from everyone everywhere. They'll figure it out sooner or later if you let them build the infrastructure now.
They currently are age-gated. The age gating clearly isn't working. Advertisers in the UK are not allowed to target children with alcohol or gambling ads.
I don't care how it happens, but showing alcohol ads to children in apps for children because the parent has a credit card and owns the phone is sub-optimal.
I'm not trying to be difficult, I really think that it's the parents' responsibility. And if ads are considered harmful in your household, don't use ad based apps / use an ad blocker? That should solve the problem on a local level.
Edit: It just came to my mind that ad supported apps are often/usually free.. Is that the case here? Because I seriously dislike the idea of blaming a developer of a free application for different ideas about morality or a bad choice ('copy/paste some ad integration code' - stupidity vs malice?) in implementing a way to make some money.
If you have a random app (unrelated, not a porn site) showing ads (unrelated unless a porn site) of alcohol services (not sure what that is, but .. unrelated/not porn), how does this apply?
How would you age-gate ads? Wouldn't that require a new X-KID-PRESENT: 1 http header so that each and every ad network can suppress ~questionable~ content? In fact: That's a great thought experiment: Is a site that serves ads that might contain 'adult' ads at times a porn site according to the British government and would need to verify that the visitor is of age?