Adult

Biologically, an adult is an organism that has reached sexual maturity. In human context, the term "adult" has meanings associated with social and legal concepts. In contrast to a "minor", a legal adult is a person who has attained the age of majority and is therefore regarded as independent, self-sufficient, and responsible. The typical age of attaining legal adulthood is 18, although definition may vary by legal rights, country, and psychological development.
Human adulthood encompasses psychological adult development. Definitions of adulthood are often inconsistent and contradictory; a person may be biologically an adult, and have adult behavior but still be treated as a child if they are under the legal age of majority. Conversely, one may legally be an adult but possess none of the maturity and responsibility that may define an adult character.
In different cultures there are events that relate passing from being a child to becoming an adult or coming of age. This often encompasses the passing a series of tests to demonstrate that a person is prepared for adulthood, or reaching a specified age, sometimes in conjunction with demonstrating preparation. Most modern societies determine legal adulthood based on reaching a legally specified age without requiring a demonstration of physical maturity or preparation for adulthood.
Biological adulthood.
Historically and cross-culturally, adulthood has been determined primarily by the start of puberty (the appearance of secondary sex characteristics such as menstruation and the development of breasts in women, ejaculation, the development of facial hair, and a deeper voice in men, and pubic hair in both sexes). In the past, a person usually moved from the status of child directly to the status of adult, often with this shift being marked by some type of coming-of-age test or ceremony. During the Industrial Revolution, children went to work as soon as they could in order to help provide for their family. There was not a huge emphasis on school or education in general. Many children could get a job and were not required to have experience as adults are nowadays.
After the social construct of adolescence was created, adulthood split into two forms: "biological adulthood" and "social adulthood." Thus, there are now two primary forms of adults: biological adults (people who have attained reproductive ability, are fertile, or who evidence secondary sex characteristics) and social adults (people who are recognized by their culture or law as being adults). Depending on the context, "adult" can indicate either definition. Another aspect of adulthood that is relatively new is psychological functioning. According to Jeffrey Arnett, adults can make financial decisions on their own, are financially independent, and are able to make difficult life decisions on their own.
Although few or no established dictionaries provide a definition for the two word term biological adult, the first definition of adult in multiple dictionaries includes "the stage of the life cycle of an animal after reproductive capacity has been attained". Thus, the base definition of the word adult is the period beginning at physical sexual maturity, which occurs sometime after the onset of puberty. Although this is the primary definition of the base word "adult", the term is also frequently used to refer to social adults. The two-word term "biological adult" stresses or clarifies that the original definition, based on physical maturity (i.e having reached reproductive competency), is being used.
The time of puberty varies from child to child, but usually begins around 10 or 11 years old. Girls typically begin the process of puberty at age 10 or 11, and boys at age 11 or 12. Girls generally complete puberty by 15–17, and boys by age 16 or 17. Nutrition, genetics and environment also usually play a part in the onset of puberty. Girls will go through a growth spurt and gain weight in several areas of their body. This is due to the natural processes of puberty, but genetics also plays a part in how much weight they gain or how much taller they get.
Emerging science on brain development suggests that most people don't reach full maturity until the age of 25. Critical parts of the brain that perform decision-making are not fully developed until around that age. For example, the prefrontal cortex, which inhibits impulses and helps plan and organize behavior to reach a goal, is not yet fully developed at age 18. The reward system of the brain also gradually reaches a stable level at around age 25.
Legal adulthood.
Legally, "adulthood" typically means that one has reached the age of majority – when parents lose parental rights and responsibilities regarding the person concerned. Depending on one's jurisdiction, the age of majority may or may not be set independently of and should not be confused with the minimum ages applicable to other activities, such as engaging in a contract, marriage, voting, having a job, serving in the military, buying/possessing firearms, driving, traveling abroad, involvement with alcoholic beverages, smoking, sexual activity, gambling, being a model or actor in pornography, running for President, etc. Admission of a young person to a place may be restricted because of danger for that person, concern that the place may lead the person to immoral behavior, or because of the risk that the young person causes damage (for example, at an exhibition of fragile items).
One can distinguish the legality of acts of a young person, or of enabling a young person to carry out that act, by selling, renting out, showing, permitting entrance, allowing participation, etc. There may be distinction between commercially and socially enabling. Sometimes there is the requirement of supervision by a legal guardian, or just by an adult. Sometimes there is no requirement, but rather a recommendation.
Using the example of pornography, one can distinguish between:
With regard to films with violence, etc.:
The age of majority ranges internationally from ages 15 to 21, with 18 being the most common age. Nigeria, Mali, Democratic Republic of Congo and Cameroon define adulthood at age 15, but marriage of girls at an earlier age is common.
In most of the world, including most of the United States, India and China, the legal adult age is 18 (historically 21) for most purposes, with some notable exceptions:
Social construction of adulthood.
In contrast to biological perspectives of aging and adulthood, social scientists conceptualize adulthood as socially constructed. While aging is an established biological process, the attainment of adulthood is social in its criteria. In contrast to other perspectives that conceptualize aging and the attainment of adulthood as a largely universal development, regardless of context, nation, generation, gender, race, or social class. Social scientists regard these aspects as paramount in cultural definitions of adulthood.
Further evidence of adulthood as a social construction is illustrated by the changing criteria of adulthood over time. Historically, adulthood in the U.S. has rested on completing one's education, moving away from the family of origin, and beginning one's career. Other key historical criteria include entering a marriage and becoming a parent. These criteria are social and subjective; they are organized by gender, race, ethnicity, social class, among other key identity markers. As a result, particular populations feel adult earlier in the life course than do others.
Contemporary experiences of and research on young adults today substitute more seemingly subjective criteria for adulthood which resonate more soundly with young adults' experiences of aging. The criteria are marked by a growing "importance of individualistic criteria and the irrelevance of the demographic markers of normative conceptions of adulthood.":230 In particular, younger cohorts' attainment of adulthood centers on three criteria: gaining a sense of responsibility, independent decision-making, and financial independence.
Jeffrey Arnett, a psychologist and professor at Clark University in Massachusetts, studied the development of adults and argues that there is a new and distinct period of development in between adolescence and adulthood. This stage, which he calls "emerging adulthood," occurs between the ages of 18 and 25. Arnett articulates five distinct features that are unique to this period of development. This stage is an age of identity exploration, instability, self - focus, feeling in between, and an age of possibilities. Arnett describes these individuals as able to take some responsibility for their lives, but still not completely feeling like an adult. Arnett describes individuals in this stage of development as deciding who they are and what they want out of life. Often times they change residences because they are getting higher education and are no longer tied to the routine of living with their parents. Arnett says these individuals can essentially go where they want when they want. Arnett also says that "optimism reigns." People in this stage of life are confident that they can make a better life for themselves than their parents had and can have deeper, longer lasting relationships.
Religion.
According to Jewish tradition, adulthood is reached at age 13 (the minimum age of the Bar Mitzvah or Bat Mitzvah) for Jewish boys and girls; they are expected to demonstrate preparation for adulthood by learning the Torah and other Jewish practices. The Christian Bible and Jewish scripture contain no age requirement for adulthood or marrying, which includes engaging in sexual activity
The 1983 Code of Canon Law states, "A man before he has completed his sixteenth year of age, and likewise a woman before she has completed her fourteenth year of age, cannot enter a valid marriage". According to "The Disappearance of Childhood" by Neil Postman, the Christian Church of the Middle Ages considered the age of accountability, when a person could be tried and even executed as an adult, to be age 7. While certain religions have their guidelines on what it means to be an adult, generally speaking, there are trends that occur regarding religiosity as individuals transition from adolescence to adulthood. The role of religion in one's life can impact development during adolescence. The National Library of Medicine (NCBI) highlights some studies that show rates of religiosity declining as people move out of the house and live on their own. Often times when people live on their own, they change their life goals and religion tends to be less important as they discover who they are. Other studies from the NCBI show that as adults get married and have children they settle down, and as they do, there tends to be an increase in religiosity. Everyone's level of religiosity builds at a different pace, meaning that religion relative to adult development varies across cultures and time.