When creating societies one of the most important things is to think about the basic assumptions it runs on, and to make sure that I am not just projecting the way we do things here and now as the only way to do things.
Like every other element of cultures what adolescence means changes, from one generation to the next with massive ramifications.
For most of human history the concept of adolescence as we know it did not exist. You were a man or women as soon as you could do the work of an adult, before which you were taught by helping the adults.
In several societies there was some kind of test or rite where you entered adulthood. In some cases it was just recognizing you. In others it was a full on test to see if you were ready to take the responsibilities of being a full member of society.
Remember that when you set an age and call anyone below that a child and anyone over an adult you are making a generalization. Some below that number are going to be ready and will only be held back, some older are not ready and won’t be for years. Your picking a spot in a field of gray and calling one side white and one side black.
We do it because there is no other way to do it, at least that we will accept.
However just because we do it that way does not mean that is the way it always will have to be done. And of course even if our way is the best way that does not mean that someone in the future will or will not get it wrong.
Even from nations that could trace themselves to us there are several reasons the idea of adulthood would change.
The basic idea of people having to be younger or older to be adults is something that even in far future sci-fi you rarely see, which is a shame. Not to say that every reason it would change would be good of course.
The most obvious reason is that more work is needed to keep things running and the life expectancy is lower. If a society is on the verge of starvation they might not be able to afford to keep people who are physically and mentally capable of helping from doing so.
In a sci-fi setting this could mean anything from farming to crawling along the inner hull dragging wires behind you.
What if you lived on an asteroid and had to be self-sufficient? It could easily take almost all the manpower of everyone just to stay alive. On earth a great many things are provided for free: lack of radiation, air, gravity to name a few. If you had to fight to get all of those, and everyone had to pull some shifts getting dosed with radiation they might not often live past 35.
What might it be like to dock and encounter a sixteen year old cargo master who is directing people a few years younger then him to move cargo around?
It might be easy to have it set up with a few assholes in charge but it needant be that way. You could have them even proud of what they have done and what they are building. How would sixteen year old act if they were considered adults and had been for years? If they were trusted with the lives of others and trusted others with their lives?
One of the practical reasons for adulthood being 20ish is education, it takes until 18 or so to get through high school in the US, and can take four years or longer to get a collage degree.
A large part of the reason for this is because it is regimented, for the most part everyone goes through it at the same pace. This leads to inefficiencies. Ones that might not exist in the future.
One example would be if you had a computer that taught people with the skill and efficiency of a twenty four hour tutor who was an expert on everything. How fast might you progress? Could you have people five years ahead of where they would be now? If so how would that effect when they are considered adults?
One almost cliche answer is to download knowledge into peoples heads, after all a very large amount of what doctors and lawyers do in school is memorize. If you did not have to do that how fast might you be able to start working?
What would you do if you were sixteen and had all the education you needed to do your job? Would you be happy just sitting around or would you want to go out and start your adult life?
Teenage rebellion and outbursts are commonly a source of problems, and while most of the attempts to deal with it impose restrictions, what if a people did the opposite? What if they decided that you were given any adults rights you wanted but you had to earn them?
What if there were exit tests as the sole arbiter of exiting high school, tests that you had a right to take at any time.
Maybe even requiring people to hold a job for so long.
This of course could turn very bad, imagine if people set out with the best intentions and placed tests to become adults in the eyes of the law, each stage granting you more rights and responsibilities.
What if it were undermined, made harder and harder to the point of impossibility. What kind of problems would that cause? Would you have a de facto caste system spring up almost over the course of a generation? People who can’t pass a test so they can’t own property or travel freely?