A birthday is the anniversary of the birth of a person, or figuratively of an institution. Birthdays of people are celebrated in numerous cultures, often with birthday gifts, birthday cards, a birthday party, or a rite of passage.

In most legal systems, one becomes designated as an adult on a particular birthday (usually between 12 and 21), and reaching age-specific milestones confers particular rights and responsibilities. At certain ages, one may become eligible to leave full-time education, become subject to military conscription or to enlist in the military, to consent to sexual intercourse, to marry with parental consent, to marry without parental consent, to vote, to run for elected office, to legally purchase (or consume) alcohol and tobacco products, to purchase lottery tickets, or to obtain a driver's licence. The age of majority is the age when minors cease to legally be considered children and assume control over their persons, actions, and decisions, thereby terminating the legal control and legal responsibilities of their parents or guardians over and for them. Most countries set the age of majority at 18, though it varies by jurisdiction.

In sociology, socialization is the process of internalizing the norms and ideologies of society. Socialization encompasses both learning and teaching and is thus "the means by which social and cultural continuity are attained".

Socialization is strongly connected to developmental psychology. Humans need social experiences to learn their culture and to survive.

Socialization essentially represents the whole process of learning throughout the life course and is a central influence on the behavior, beliefs, and actions of adults as well as of children.

Socialization may lead to desirable outcomes—sometimes labeled "moral"—as regards the society where it occurs. Individual views are influenced by the society's consensus and usually tend toward what that society finds acceptable or "normal". Socialization provides only a partial explanation for human beliefs and behaviors, maintaining that agents are not blank slates predetermined by their environment; scientific research provides evidence that people are shaped by both social influences and genes.

Genetic studies have shown that a person's environment interacts with their genotype to influence behavioral outcomes.

Positive socialization is the type of social learning that is based on pleasurable and exciting experiences. We tend to like the people who fill our social learning processes with positive motivation, loving care, and rewarding opportunities. Positive socialization occurs when desirable behaviours are reinforced with a reward, encouraging the individual to continue exhibiting similar behaviours in the future.

From the late 1980s, sociological and psychological theories have been connected with the term socialization. One example of this connection is the theory of Klaus Hurrelmann. In his book Social Structure and Personality Development, he develops the model of productive processing of reality. The core idea is that socialization refers to an individual's personality development. It is the result of the productive processing of interior and exterior realities. Bodily and mental qualities and traits constitute a person's inner reality; the circumstances of the social and physical environment embody the external reality. Reality processing is productive because human beings actively grapple with their lives and attempt to cope with the attendant developmental tasks. The success of such a process depends on the personal and social resources available. Incorporated within all developmental tasks is the necessity to reconcile personal individuation and social integration and so secure the "I-dentity". The process of productive processing of reality is an enduring process throughout the life course.

The problem of order or Hobbesian problem questions the existence of social orders and asks if it is possible to oppose them. Émile Durkheim viewed society as an external force controlling individuals through the imposition of sanctions and codes of law. However, constraints and sanctions also arise internally as feelings of guilt or anxiety. If conformity as an expression of the need for belonging, the process of socialization is not necessarily universal. Behavior may not be influenced by society at all, but instead, be determined biologically.