The right to make one’s own legal decisions is typically denied for two reasons: Youth (minority) and incompetence. Incompetence is a state of mental incapacity to make one’s own decisions in a way which is informed with regard to the consequences and rational.
Note that, for a decision to be made competently, there is no requirement for it to be “fully” informed, only for it to be informed “enough” (for example, your decision on whether to move into an apartment needs to know the location of the apartment and the length of the lease, but you are not considered to make an uninformed decision simply because you don’t know that there was mold in the unit 6 years ago. If you find this out after moving in, this fact would probably not be considered sufficient to void the lease, as long as the mold problem was treated. This is so even if the previous problem means that there is an elevated chance of a recurrence of the problem in the dwelling.)
Similarly, your decision needs to be rational “enough”, not necessary “fully” rational. For example, you need to realize that you will have to find a new job that enables you to cover the rent if you are working a temp job that ends in 4 months and are signing a 12-month lease. However, you are not considered to be making an irrational decision if it turns out that you are willing to take a higher risk of financial problems because you just really, really think “the neighborhood is awesome”. We might call such a decision unwise, but not irrational. Many complex decisions we make in our lives are subject to cognitive biases (as any Psychology 101 class will show you!), but that is not by itself sufficient grounds to back out of a contract or declare the signing party incompetent to enter the contract, in the absence of circumstantial considerations.
When it comes to youth (minority), there is a presumed incompetence of young persons to make an informed and rational decision. Minority, or non-age, is the state of being below the age of majority, typically 18, a traditional age at which one is assumed to have the mental capacity sufficient for informed decision-making independent of parents or guardians. However, it is far from clear what grounds this age should be based on. Many commentators, even when evidence is presented to the contrary, argue that adolescents (for the sake of this discussion, ages 13-17) are not fully developed in their decisional capacities, or equivalently, that their decision competence is measurably less than that of adults, and thus that the age of majority needs to be set at the end of adolescence instead of at the beginning, and that until the age of 18, young people are not usually able to be responsible decision makers regarding matters such as medical procedures .
However, upon closer examination, this is actually a double standard when compared with the notion of legal incompetence. This is so because competence is only supposed to require a decision to be informed “enough” and rational “enough”, not “fully” informed and “fully” rational. This is inconsistent on grounds of principle with the doctrine that one should remain a minor until their decisional capacity is “fully” developed and mature, rather than developed “enough”.
Of course this raises the obvious question of how to determine when a person (or birth cohort) is developed “enough” to be given the right to self-determination. Once we manage to rid ourselves of the notion that a person needs to be “fully” mature or developed, how do we set a benchmark?
The most parsimonious and egalitarian approach should follow the principle (I’ll call it the “parity” principle):
Two persons, having the same abilities in the relevant sense, should be given the same right to self-determination, when it is practical to do so without imposing an undue burden on the public to assess the abilities. In the case of the right to vote, it is to be understood that performing individual exams to assess maturity or competence may not be reasonable due to the risk of politically motivated conflicts of interest with regard to the construction of the assessment.
We also must consider that any assessment that could be given is, in reality, imperfect, as is any empirical category a person may fall in (adolescent, adult, black, white, female, male, stroke patient, concussion victim, war veteran, professor, clerk, etc.). Almost no assessment score or category tells us with 100% certainty whether a person is incompetent or competent. Thus we have to ask the following question: If due to age, other characteristic, or a test result, a person has a 50% chance of being competent, do we give them the right to self-determine? What about if it is 30%? Or 70%? This gives rise to the “statistical parity” principle:
Two groups of people, in the absence of further information, both containing the same fraction of competent persons, in the relevant sense, should be given the same right to self-determination, when it is practical to do so. For example, if 20% of people with medical condition X are incompetent, and 20% of people with medical condition Y are incompetent, we should either allow both the right to self-determine, or deny both that right, pending/in the absence of further determination by a doctor or otherwise. Similarly, if 20% of people at age Z are incompetent, they should be treated the same as those with medical condition X or Y.
But we still have to actually compare persons in these differing groups quantitatively in order to assign an appropriate age of majority. Where do we start? In the spirit of fair use, I will bring in the data for the legal competence to stand trial as measured by a test designed by the MacArthur Foundation . The reason for bringing in this example is that there does not seem to be any good reason to think that the mental capacity necessary to make this type of decision is dramatically different from the capacity necessary for other types of real-world decisions (legal, medical, etc.) so it is a reasonable proxy for decision-making in a somewhat more general context.
In this study, there are three main areas of assessment: Understanding, Reasoning, and Appreciation. The youngest age group, 11-13 years of age, scored 10.45 (3.31), 11.30 (2.82), and 9.68 (2.34) respectively, given as mean(standard deviation). The oldest (adult) group scored, respectively,
12.13 (2.92), 12.57 (2.51), and 10.77 (1.57) on these areas. (See Table 3 of that source.) Of note is the fact that the average person in the 11-13 year age group is actually within the normal range of the adult population, that is, the population mean for the 11-13 year olds is less than one standard deviation below the adult mean in all three areas. This means that to have an age of majority greater than 13 years of age, according to this study, would violate the Parity Principle unless a sizeable percentage of the adult population were also declared incompetent. Since the latter outcome is absurd, it seems that this suggests that 13, not 18, is a reasonable choice for the age of majority. One may object that while the average 13-year-old is competent based on this result, the age of majority must be higher to protect those with below-average decisional capacity and maturity levels. The problem with this argument is that it would essentially restrict the entire group of young people in order to guard against the result of an incompetent minority within the group. We do not do this for stroke patients or people with a brain injury; legally on the basis only of the fact of stroke or brain injury these people cannot be declared incompetent. A finding of incompetence would require more than this – the assessment of one or more qualified professionals. Thus the idea of non-emancipation of adolescents in order to protect the minority is a violation of the Statistical Parity Principle, since a significant fraction of those with strokes and brain injuries are in fact incompetent to make decisions for themselves, and yet we do not use this “protect the minority” reasoning for them.
It may be objected on the grounds of some recent research that “raw” cognition reaches adult-like values sooner than psychosocial maturity  . This is based on a number of sub-studies outlined in Table 2 of this source. One of these sub-studies  has a set of data presented in a manner similar to the MacArthur study data mentioned earlier. In this study, the “resistance to peer influence” for the community sample of 13-year-olds is 3.09 ( 0.48) and for the 23-year-olds (adults) is 3.46 (0.43). Even though the age at which fully adult values is attained is fairly high, the mean for the 13-year-old group is within one adult standard deviation of the adult mean. So what is purported to suggest against a lower age of majority, upon closer examination, actually supports a lower (13 years) age of majority, once one uses the Parity Principle and the Statistical Parity Principle. This is true even for slightly younger kids based on these numbers, however the same cannot necessarily be said of other studies. The support for lowering the age of majority to 13 years of age requires one more thing, which I will call the Adult Variation Equivalence Hypothesis:
The typical 13-year-old, on any reasonable measure of social, emotional, or cognitive development or maturity, if evaluated by adult standards, is within the normal range for adults, which, inclusively speaking, is that between the 5th and 95th percentile for the adult score on any assessment.
The challenge to those who want to argue against lowering the age of majority to 13 years of age, is to demonstrate an example case where the Adult Variation Equivalence Hypothesis does not hold. To the author’s best knowledge, there are no known exceptions.
It may be objected that youth lack some necessary amount of “life experience” to be competent at being the ultimate decision-maker for themselves; however the question arises as to why this lack of life experience does not seem to result in any corresponding deficits in judgment capacity as measured by any of the aforementioned sources. Furthermore, if a lack of “life experience” were so crippling to a person’s judgment, we should expect to see older people (let’s say, over age 50 or so) have outstanding success in areas of life that require complex social skills, and we would expect to see younger people, even past the age of 18, have problems relative to those in later decades of life. Yet this is not true in all fields. For example, many of the most successful businesses in the modern world were started by people in their younger years, demonstrating that they possess very powerful social skills at least as often as those at more advanced ages. This should throw serious doubt on, if not outright refute, claims that one needs a large amount of accumulated “life experience” to be a competent member of society. The empirical data on young people’s judgment and the age distribution of entrepreneurs suggest no detectable “life experience” effect operating independently of cognitive development. Therefore it seems warranted to consider only cognitive development and decisional judgment in various scenarios as relevant, and that the number of years of accumulated “life experience” is at most an insignificant factor .
The author makes the bold, radical suggestion that 13 years is the proper choice for the legal age of majority. This is further supported by bioevolutionary considerations (puberty). The author is not suggesting that the parental financial duties end at that age until the education system is reformed to allow for it. In the meantime, adolescents should be treated in the same way as adults who are not financially self-sufficient for other reasons but who are nonetheless competent (e.g. injury, unemployment, stay-at-home-parent, divorcee claiming alimony).
 (Hodgson v. Minnesota, 497 U.S. 417 (1990) ).
 (Grisso, Thomas, Laurence Steinberg, Jennifer Woolard, Elizabeth Cauffman, Elizabeth Scott, Sandra Graham, Fran Lexcen, N. Dickon Reppucci, and Robert Schwartz. "Juveniles' Competence to Stand Trial: A Comparison of Adolescents' and Adults' Capacities as Trial Defendants." Law and Human Behavior 27.4 (2003): 333-63. Univ. of California, Riverside. Web. 16 May 2017. <https://stopyouthviolence.ucr.edu/pubs_by_topic/5.Juveniles'%20competence%20to%20stand%20trial.pdf>.).
(Steinberg, Laurence, Elizabeth Cauffman, Jennifer Woolard, Sandra Graham, and Marie Banich. "Are Adolescents Less Mature than Adults?: Minors' Access to Abortion, the Juvenile Death Penalty, and the Alleged APA "flip-flop."" American Psychologist 64.7 (2009): 583-94. Univ. of Colorado, Boulder. Web.)
 (Steinberg, Laurence, and Kathryn C. Monahan. "Age Differences in Resistance to Peer Influence." Dev Psychol. 43.6 (2007): 1531-543. U.S. National Inst. of Health. U.S. National Library of Medicine, 19 Nov. 2009. Web. 16 May 2017.)