Premising that I particularly appreciated @KPFerguson's post, I'm more for a (still fixed) progressive way, not thinking to "a day" but to "a range": starting from a minimum legal age of 16, younglings should be progressively involved into being more responsible, in different fields, year after year depending on the field, up to the "full legal age" which, I agree, should be 25.

So, it's a matter to decide what they're responsible for, at the different legal ages. Let me try some few examples:

1. 16: drive a lightweight car/motorbike, to have a job,
2. 18: to vote, to marry, to parent, to drive bigger cars, little astro-ships or mining cargos ;-)
3. 21: to be elected in Parliament⁽¹⁾,
4. 25: to be elected in Government⁽¹⁾,

⁽¹⁾ I know that the Constitutional draft states differently, but this is just an example.

This way, a person will have the time to "receive the burden" of being an active part of the society, without having to cope, from a moment to the other, to the full impact of that like it is, now, having many countries a static legal age of 18. This, not only, as correctly stated by @KPFerguson, gives the family a "fixed point" of reference, to plan the young man development, but gives the family more time also, than having to do "all for the 16th birthday".

Just go by mental ability and education. Once you graduate the basic Asgardia education system you are considered adult at the moment of graduation. If you are smart enough and mature enough to graduate sooner then you are considered adult sooner.

I'm sure every citizens education record will be logged and able to be reviewed for a "maturity" assesment.

@Protean

What would you propose to determine the legal age of majority?

Okay. Then what is the appropriate age of majority?

Wasn't asking what the constitution declared. I was asking for your opinion.

When we are talking I assume that the Age of Majority is the age when the citizen is able to be subjet of rights and obligations. Due to the dificult taks of assigning ages of majority to each right or obligation  I agree with the decision of the Age of Majority. But Which age should we take in to account? As we are all citizens of the Earth that age should be between 18 and 21 due to the fact that the conscience of the person is completly developed.

This is so simple it hurts constitutions article 8-1 paraphrased Asgardia fallows all international laws  

U.N. Convention on the Rights of the Child 1989  

The  Convention on the Rights of the Child (CRC) is the most comprehensive document  on the rights of children.[7]   Based purely on the number of substantive rights  it sets forth, as  distinct from implementation measures, it is the longest U.N.  human  rights treaty in force and unusual in that it not only addresses the   granting and implementation of rights in peacetime, but also the  treatment of  children in situations of armed conflict.   The CRC is  also significant because it enshrines, “for the first time in  binding  international law, the principles upon which adoption is based, viewed   from the child’s perspective.”[8]   The CRC is primarily concerned with four  aspects of children’s rights  (“the four ‘P’s”): participation by children in  decisions affecting  them; protection of children against discrimination and all  forms of  neglect and exploitation; prevention of harm to them; and provision of   assistance to children for their basic needs.[9]   For the purposes of the CRC, a child is  defined as “every human being  below the age of eighteen years unless under the  law applicable to the  child, majority is attained earlier” (article 1).  

boom theirs your answer.

As a 15-Year-Old attending Early College and who is wanting to be more involved in my local community, I will propose my idea concerning the Youth's involvement with voting. I think that instead of allowing the Youth to vote, there is an alternative. Forming a committee that consists of only the youth, however, is supervised and there could be one or two children who could take on a leadership role that works closely with the supervisor and represent the committee. The committee members could be chosen through an application process and the one or two children should only be chosen or considered to be a representative if they've been apart of the committee for an "X" amount of time and should be voted in by the Youth. The committee should come up with the Qualifications required for this role. The committee's purpose would be to discuss any issues brought forth to them or any issues they feel are important and worthy of discussion to try to give their input or possibly provide a solution. They would then come to a consensus and put out an official report of their Majority opinion regarding the solution or advice, etc. alongside with a separate document containing any and all unresolved issues. These issues would be sent to the proper officials or to the person/group the document regards. The Youth's committee's amount of power would still require further discussion because as of now, I am unsure about the extent of the committee's powers. I know there are still many things to figure out before a committee like this is set up or even considered, but please consider it. If you would like to discuss the uncertains that still remain regarding this idea, then please respond and we could work on solving these. Thank you for your time.

I agree with Christian 42. I believe there should there should be a Youth Parliament, to work with the older parliament. But what is necessary to this topic is To make sure how we teach in asgardian education so we can actually determine the perfect age to allow allow to participate in elections. The reason why not as many minors participates in politics is because they aren't taught as much politics as for the unnecessary subjects they take today. As older individuals we tell our youth they have no right to say anything because they "don't know any better". Instead we should be telling them "You don't know now but you will soon because you have the right to shape this nation into a greater image, and we'll make sure you know as mush as we do".  The reason why don't know any better is because most people look down on their potential, I believe we shouldn't limit their freedom of speech so much as how the U.S.A has and instead to work along them. To figure out the flaws we don't see but they do. Once we figure out this we might have a clearer understanding of the "legal age" for anything.

I am actually fairly perplexed at some answers on both ends of the spectrum.  To those of you who would desire to make the age of legal maturity be 25, that would mean spending a third of your life, and well into your physical maturity as an adult, being under your parent's supervision, and not being allowed to take responsibility for your own actions.  Sure, many people in their early twenties are obviously not as responsible or mature as those in their late twenties, thirties, or older.  But I would actually posit that that has more to do with life experience, they are finding their way in the world, meaning that if you pushed back this age further, it would simply delay even further the emotional maturity of these individuals.  Except this would actually come with serious drawbacks, because biologically and psychologically, by your late twenties and early thirties, your brain has begun to more fully cement itself into your established identity, and if you were never given an opportunity to be an adult until this time, it is too late to learn how to be a responsible adult.

To those of you who think the age of voting should be 14, I highly doubt that any one that age has -- or is even capable of the emotional maturity to make decisions regarding the direction a society or entire community should move in.  One of you said, most people that age already know how they would vote.  But from what I remember being that age, everyone is just forming opinions for the first time about things like politics, and they are highly impressionable, so in general they end up parroting and repeating the views of their parents.

And to those who think it should simply be determined on an individual basis, if you are serious about establishing a nation which occupies a contingent space, I would say that a community in general should move along side of each other.  One user put it quite well, I thought, when they said that parents should know how long they have until their child is an adult, so that they (and their teachers) can plan and schedule the moral and educational development of the children.  If a teenager was actually able to pass the test at the age of 15, for example, but highschool goes until the age of 18, would they get to drop out and be an adult then?

What I would be in favor of would be a (slightly) younger age of legal maturity and legal responsibility, and a (slightly) higher age for voting.  I would propose an educational system which graduates secondary education (highschool) around the age of 16 (on an accererated track above most western countries), followed by a mandatory two year's "gap year" program for them to join the work force, especially on either an internship or apprenticeship, during which time they could either continue living with their parents or move into their own apartment or with room mates.  Then at the age of 18, they could either voluntarily start tertiary education (university), or they could continue to work.  The reason for this "gap year" program is two-fold.  First, I believe maturity and wisdom comes ultimately from life experience, and this would help young people learn about not only themselves, but also about the way that real life (adult life) in the community actually operates.  Secondly, there seems to be a huge phenomenon in many developed countries where kids go straight from highschool into college, with no work experience or ideas about what kinds of jobs they would actually enjoy or be passionate about.  And yet, they are pressured to decide then what they want to dedicate the rest of their life towards, and often choose either something which sounds promising but they end up having no passion for, or they study something highly impractical and can't get a job in the field.  Also, given that this will be a relatively small nation existing primarily on a space station, it will likely be a very safe community for them to start living on their own at a younger age like 16.  I would be in favor of a drinking age of 16 also, and the legal age to marry I would say is 18 for a self-elected marriage, but with both the approval of their parents and a judge, they could marry at 16 (which, given those same stipulations, is actually legal in some US states).  

When it comes to voting, however, I would actually be in favor of a slightly higher age for voting.  Originally in the whole of the English-speaking world, the voting age was 21, but this was lowered to justify sending 18 year olds to war.  But I would be in favor of a voting age around 21.  Like I said previously, I strongly believe that maturity comes not from years, but from life experience.  By this point, these voters will have five year's experience living and working on their own, and understand the basics of daily life and the reality of taking part in the economy, and thus would be more entitled to political opinions.  Some of you have said an age of 25 would be warranted.  Most of you gave no reason why you believed this, but one person cited that this is supposedly when the psychological development of a person is fulfilled.  Simply in terms of voting, as long as all other rights were granted well before that, I would concede this point (which is saying something, as I am only 23).  The world of politics, even in a small nation like Asgardia, requires serious contemplation and emotional maturity.

And finally, for some reason, a select few of you are talking about the legal age for driving in Asgardia.  Are you kidding me?  You want to go live on a space station in Earth orbit, and then fill it with cars, city streets, and freeways?  That is just so impractical, on so many levels.  Even if Asgardia were big enough to warrant complex infrastructure of that sort, it would be much more likely to take the form of lifts and elevators, or perhaps even some system resembling a subway or metro rail.  And there is no way a teenager is operating a space shuttle or transport outside of the station.  Those kinds of systems would require much more intensive training than even flying a plane, much less driving a car.

I was in the 23-25 camp before comments on a Facebook thread.
After that I changed to supporting 18. Here is my blog post on it:

https://asgardia.space/en/blog/22020-elections-are-self-selecting-referendum-on-the-age-and-maturity-of-candidate/

In my opinion, 18 is the best age for majority. The reason is because this is the legal age for some of the majority countries in the world.