The treatment of members of our society that are not (yet) vested in it owing to a lack of sufficient immersion and mental development shape the means with which we should treat them. This generally applies to children as a temporary condition, but it also applies to those who are physically adult but who through some generally biological lacking do not have the capacity to develop to a state within a certain lower bound of normal mental capacity. We recognise the state of being mentally capable to act within the limits of society as being of Majority, and being in a lesser state than that as requiring one to be a ward of some person or institution, in the first (and sometimes the second) case with the intent of preparing them for that status and responsibility and in both for protecting them and others, within reasonable limits and expectations, from some of the consequences of their interactions. A person of majority is given the greatest responsibility and autonomy for their actions consistent with the public good, while a person not of majority is given more limited responsibility and autonomy. The actions of a person of autonomy are thus defined as having full personal weight, while those of persons not are defined as having lesser personal weights.
A person approaching majority from youth from within society must meet several qualifications. First, neurologically the brain is not fully developed until a certain age (I believe developmental neuropsychology places this age at about 19) - choosing a minimum age near this period is prudent. Even though it is possible to show most characteristics of advanced intellect at an early age for exceptional people, the full judgement of a mature brain will not generally be present even for a brain with much raw talent, and the threshold is actually a dual-one, intended to both meet a minimum mental function required to be a reasonably competent member in society and to meet a minimum threshold of one's individual capacity. Second, the person must have a biological capacity for empathy - some rare mental disorders drastically reduce or eliminate this, and it is possible to stunt this through unusual upbringing. Third, the person must have been raised in a way that, very broadly speaking, allows them to function in society. Very poor or unusual upbringing may in some circumstances limit their ability to understand society, interpersonal norms, and similar - people raised in a cloistered environment with highly nontraditional norms (e.g. Taliban-style Sharia) should not be immediately eligible for majority.
Those in minority have restrictions and protections in place designed to more actively protect them and others from the consequences of them not being fully vested in society. Being in minority may have several levels, defining and distinguishing those being outside the scope of this concept. First, minors have a more limited ability to contract and conduct transactions, being able only to act as proxies for such transactions at the lowest levels (e.g. parents giving their children money to buy money in a market or lunch money) and at the very least restrained from large transactions and commitments with significant duration. Second, minors should have lesser liability from their actions that are against laws, particularly those that have limited or no actual harm and are more designed to provide order than protect from great harm (their guardian may have fines or liability placed against them as incentive to impress on the minor the importance of not violating these norms, although this should be used lightly). Third, minors should be recognised by members of the community as not being fully responsible and in legal grey areas, particularly when there is no danger involved to members, treated with some light duty of care. Fourth, minors should not be treated as yet having full personal autonomy, being bound to their guardian, potentially steered free from potentially troublesome parts of society by protective laws specific to them, and having some measures in place designed to protect them from the long-term consequences of their (and others) actions. Broadly, large, unusual, and not-easily-reversed changes in personal status should not be permitted of minors - marriage, many instances of cosmetic surgery, female circumcision, implants of various kinds, any kind of contract that would survive the transition to majority, duels, joining a military, etc. Decisions to do these matters are not meant to be relegated to the guardian - they are changes that are personal and should be made only by those in majority for themselves.
A transition to joining the majority should be come when it is clear to the community that the requirements to act as such are met. It is acceptable to have different levels of majority, although in such cases it would be useful to the community through some means to be able to tell what level of duty/care someone belongs in, particularly if they are travelling unaccompanied by a custodian. At time of a transition to custody, a person should have set aside for them by (or obliged on need from without conditions) their former guardian sufficient funds to survive for a time sufficient to find employment and support themself (even if their immediate plan is to prolong their dependence, e.g. attending university). Additionally, they should be free of all debts and not under any legal or strong social obligations (e.g. marriage or military service unless it is mandatory on all society).
Under ordinary circumstances, a minor will be under custody of their parent(s) and/or extended family, provided they are and remain willing and able to exercise their duty as a guardian. Society has a reserve responsibility (read: reserve power) towards them, and if their guardians fail, society may assist, adjust, or terminate their relationship with whatever guardians they have, assigning supplemental or replacement guardians if and as needed. The cost and strife inherent in this along with the welfare of the minor should be considered, and financial obligations may be placed by society onto their former guardians if appropriate for future guardianship (particularly blood ties, and in these cases if parents have not exercised reasonable effort to properly care for a child and have custody removed this way, they may be enjoined from having other children). A minor may be a recipient of a bequest or gift, and a guardian may not, unless the bequest specifically allows them to do so or the guardian lacks significant means (or is the state itself) and the bequest or gift is large, use it to offset the cost of raising them.
Those entering society from another may need, depending on differences from the society from which they enter, time as a minor (even as an adult) with necessary acclimation. Unless a strong societal interest is served, making arrangements for this should be funded by those who wish to enter (or other willing parties) rather than society itself - society should arrange for (directly funding or requiring funds from appropriate members) its native populace when they through some accident or abuse need custody from a new source, but potential entrants should not normally recieve that and may be turned away or expelled if that need should arise.
Adults entering society temporarily from another with significantly different norms may require a temporary guardian. In this case, the guardian has a stricter liability and responsibility for that visitor than for young minors.