“We remind the court that the Second Amendment refers to a right ‘of the people’ without mentioning age, and certainly young adults fall within the definition of ‘the people’ ever since they’ve been allowed to vote, and generations before that when they were considered part of the militia, and have been accepted into the military.” [More]
They’re not just “considered.” They’re recognized by law.
“When does a child become an adult?” is a question not all that different from “When does a zygote become a human being?”
And its not going to be any easier to answer from a legal perspective.
Unfortunately, you can’t make law without definitions. Justice Potter Stewart’s “I know it when I see it.” isn’t going to cut it on this topic either.
https://history.wustl.edu/i-know-it-when-i-see-it-history-obscenity-pornography-united-states