He could take them home to their parents (and I see no reason any other adult from the community couldn't do the same thing).
Your argument would also require that a three-year-old who accidentally kills his baby brother be arrested and charged with manslaughter or murder.
And why's that?
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.
Sure, you need an age of majority for voting and consenting to sex and contracts, but not for drinking.
And why's that?
Because your essay suggested that there is no reason for a statutory age of majority, without which children and adults would be the same in the eyes of the law.