Key word is “adult.” That comes at age 18.
It was once common for folks to get married in this country (espcecially women) at 14-15 years old. This “adulthood begins at 18” is largely a post-war concept. I’m sorry, but I find it hard to believe that anyone past 15 is a “child” unless their parents didn’t wean them properly.
My new theory is that an adult is someone who manages to pay all of his or her own bills for one entire year, while not living in someone else’s house for free. Until then, don’t talk to me about what’s fair or what you deserve.