'In the US wine is sold in supermarkets but other booze in special off liceneces only.'
In some states. In others, it's all sold in a liquor store, usually located next to the supermarket .
In others, it is sold, but only 3.2% beer and wine. For the regular stuff, you have to go to a liquor store.
California and Nevada are noteable exceptions.
But of course, the liquor store isn't open as often as the supermarket, and because they only sell booze, mixers, snacks and Lotto tickets, it's easier for them to catch out underage buyers or those buying for minors.
The other issue is that, in most states, a person who buys to supply a minor can get into very, very serious trouble and even be charged the same as a perpetrator if in fact a perpetrator is a minor who was supplied by the person (in the case, say, of a minor who then drives a car drunk and kills someone).
Of course, this approach may not work for the UK, but this is how it is handled.