Most other developed countries get by just fine with a much more relaxed attitude. Demanding identification and legal documentation for every little thing is excessive and imposes a compliance burden on everyone. It's fair in the sense of being non-discriminatory, but unfair in the sense that it creates a massive aggregate burden, reduces privacy, and institutes authoritarian presumptions.
If the person manning the self-checkout operations thinks you look awfully young to be buying alcohol then they can always demand to see your ID.
Simple.
Far more harmful is self-checkout without a discount for free labor or treating your customers like criminals when they leave the store with their own property.
You seem to have missed the part where I do not want to ask permission of the government for every little thing. I'm well into middle age and do not appreciate having to seek governmental approval for what I consume. I am fine with some products like alcohol being subjected to taxes to balance out negative externalities. I am not fine with the idea of showing my papers on demand.