Why - because it's fundamentally ridiculous to tell someone they are old enough to decide volunteer for the army to go kill people and be killed overseas, but they aren't old enough to decide whether they should have a beer.
Or you can raise the age of legal adulthood to 20, for sex, nicotine, driving, voting and I'll agree with you.
It's already extortionate to go out and have a few beers at a bar. Making this more expensive at on-licenses is going to make the problem worse, as several people have pointed out.
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