If they're old enough to fight and die they should be old enough to have a drink.
First, the government has no business telling anyone, whatever his age, what substances he can consume.
So, if a police officer sees an eight-year-old drinking, smoking, or taking drugs, it's none of his business? Your argument would also require that a three-year-old who accidentally kills his baby brother be arrested and charged with manslaughter or murder.
You go from making sensible arguments, to losing all good sense. It's just common sense, that you need an official age of majority. The unavoidable imperfection of such a convention is no more a convincing argument against it, than it would be against any other social convention.