My apologies for commenting though I'm not in your class and haven't done your reading.

To me the criteria for being an adult is having the rights and expectations to the most serious of steps adults can take. Those include voting for national leadership, fighting in your nation's armed forces, voluntarily entering wedlock/having kids, owning major property, consuming restricted substances.

Different societies set that threshhold differently. Currently that tends to be 16/18 in the USA.

If society were to raise that to 21 or higher based on evidence the brain is still developing then that's fine, but then my argument would be you shouldn't be expected to or enabled to do all the adult steps I listed above until you hit that threshhold. You can't have it both ways. You can't say you're not really an adult until your 21, but oh, yeah, when you hit 16/18 you can legally do whatever you want anyway.

Well 16 yrs is probably a safe number people are comfortable with accepting. So what is the definition of an "adult" anyway. According to websters dictionary it states an adult is someone who is "fully grown or developed". Well for me and many other people this is a very vague definition for something seriously important like adulthood.

If we use factors like abilities to make money, and buy major items in life; house, car, reproductive abilities etc there are children as young as 6 years old who are doing better at some of these factors than adults. I consistently read stories of children a quarter my age making 5 times more money than me, and I am on some descent coin, so to be honest we can't use external factors to determine if a person can be considered an adult or not. Not to mention 10 year olds popping out babies, and engaging in sex exposing themselves to responsibility while being totally vulnerable and unprepared should they make a baby.

Maybe we should use internal factors like being able to make wise and sensible decisions leading to favourable consequences. If this were the case then many adults who are alcoholics, or drug addicts, or the many men and women who make babies then hit the highway as soon as possible, or theives, or adulterers, or murderers, or con artists would totally destroy this argument of "the ability to make wise decisions is the defining characteristic to determine when a person becomes an adult or not". Fact of the matter millions upon millions of people make appalling decisions daily, and people are mortgaging their homes in the desperation of getting rich quick, anyone who is sensible enough knows this is a very dangerous move and game the people are playing, what if their gamble fails (and nine times out of ten it does), the people and children lose everything, the man loses everything he worked for as well as his wife and children all because of the desire for money was the root for all their stupidity, and some by longing for it wandered away from sure steps and exposed themselves to many griefs.

I believe a kid becomes an adult when they are given a situation which is difficult to handle even in the eyes of an adult, take on the challenge presented to them and cause the situation to succeed instead of erode into chaos and destruction. History is riddled with stories of families losing the bread winner and then the eldest son steps up and takes on the responsibilities as man of the house, or parents are tragically killed and the eldest daughter steps up and nurtures her siblings towards success and taking on responsibility.

I believe you become an adult when you take on responsibilities whatever their form and shape and you steer those responsibilities and the situation towards success. Age is nothing but a number.