I'm only 19 but my mind is older.
Most of my life I have waited for adulthood. It seems like something every kid dreams about: visions of being able to do whatever you want without people telling you otherwise. Countless times, my parents would promise me the ability to do something when I'm older. They would brush it off and say "wait until you're an adult," or "maybe when you're older." This led me to a seemingly false notion that as soon as I turn 18, I would become an adult and thus be able to do what I want.
From a strictly biological perspective, adult means to have reached sexual maturity. Girls can usually achieve this by 15-17 years old and boys usually do it by 16-17 years old. This seems to mostly fit Oxford's definition: "a person who is fully grown or developed." Of course, your brain doesn't stop developing until about 25 years old.
Then there is legal adulthood. This is what I believe most people, especially those just graduating high school believe in. Most of the time, people reach legal adulthood at the age of 18. This is when a child's parents lose their parenting rights and can no longer make decisions for and in behalf of their child. The child is now legally responsible for themselves.
However, a special problem tends to arise with teens who become the age of majority. The teen mind is already one that is trying to become more independent. Because the frontal lobe is not fully developed yet, this is fuled largely out of emotion. Along with this engrained belief that they are an "adult" as soon as they turn 18, many teens will struggle with their parents at this point. "Why should my parents be able to tell me what to do if I'm an adult?"
Perhaps this emotion-fuled desire for independence is what made me so upset when I insisted that I was an adult, but my aunt said "You'll be an adult when you're married with kids." However, she might have a point. Among the things I wasn't taught in school, was the social construct of adulthood. Which, in fact, adults like my aunt seem to understand a lot better than I do.
Social adulthood is not defined anywhere. It is a subjective construct that is influenced by things such as culture, gender, race, social class, and ethnicity. Like my aunt said, marriage and parenthood are typical social markers of adulthood. Others include living away from family, completing your education, and starting a career. Social adulthood requires work. You have to put in some work to achieve markers for adulthood. If you don't others may not recognize you as such.
Perhaps these different definitions of adulthood help to explain how sometimes it feels as though people who are around my age experience parents believing they are adult enough for some things, but not for others. In the context of trying to go to the "adult" activites with relatives, I usually get shut down. Those are social activities and I probably don't fit the social definition of an adult quite yet. However, I have worked to develop applications before for Walmart. Was I adult enough for that? Yes, because I have a near adult level of knowledge required for that.
I believe that this is just one struggle that plagues college age students. We are legally adults, but not socially. So what are we? We aren't kids, we aren't adults. It is a weird limbo that's a mixture of both and honestly, it can be very frustrating at times, but perhaps that is my amygdala speaking instead of my frontal lobe.