Mechanisms of Growing Up: When Does a Person Become an Adult, and Who Is Considered a Child?

Growing up is a multifaceted transition from childhood to maturity, involving both biological transformations and psychological, social, and legal changes. Different cultures and societies may draw the line between “child” and “adult” in various ways. On one hand, there are biological markers like puberty and physical development; on the other, there is psychological maturity—emotional development, social responsibility, and self-awareness. Layered on top of these are the legal definitions of adulthood, which vary across countries, and cultural traditions that shape their own “rites of passage.”

In this article, we will examine the mechanisms of growing up in a holistic way—covering biology, law, culture, and personal self-identification. We will put a special focus on countries in the former Soviet Union (CIS) and the UAE, touching briefly on Western, Eastern, and African contexts, as well as the broader MENA region. Finally, we will analyze at what point society considers someone an adult and how that aligns—or conflicts—with each individual’s own sense of maturity.

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Growing up stages: Biological Stages of Maturation

Puberty and Physical Growth

The foundation of growing up lies in the biological changes that transform a child into an adult. Puberty (sexual maturation) is central to this process. Triggered by hormones, the body develops the ability to reproduce, secondary sex characteristics emerge, and physical growth accelerates (Puberty — Wikipedia). On average, girls enter puberty around ages 10–11 and complete it by 15–17, while boys enter at about 11–12 and complete it by 16–17 (Puberty — Wikipedia). Key milestones during puberty are the first menstrual period (menarche) for girls (approximately age 12.5) and the first ejaculation for boys (around age 13).

However, the biological process of maturing is not limited to the reproductive system. Brain development and the maturation of the nervous system continue well into the mid-20s. Research indicates that the prefrontal cortex—responsible for executive functions such as planning, impulse control, and complex decision-making—does not fully mature until around age 25 (Under the Hood of the Adolescent Brain | Harvard Medical School). As a result, even though adolescents are sexually mature by their mid-teens, full neurobiological readiness for adult responsibilities often comes later. Specialists from Harvard Medical School vividly describe the prefrontal cortex as “the last part of the brain to finish developing, well into the mid-20s,” thereby affecting a young person’s long-range planning and impulse management.

Hormonal Shifts and Emotional Turbulence

During adolescence, a surge in hormones—testosterone, estrogen, and others—fuels not only physical changes but also emotional volatility. The limbic system, which governs emotion and risk-taking, develops faster than the frontal lobes, which are responsible for executive control. This imbalance can explain the strong emotional responses and higher risk-taking behaviors often seen in teenagers (Under the Hood of the Adolescent Brain | Harvard Medical School). Over time, these biological mechanisms reach an equilibrium: by about age 20, physical growth and puberty wind down; by roughly age 25, the structural maturation of the brain is typically complete.

From a purely biological standpoint, then, “be coming an adult” is a gradual process. The body and brain “ripen” through adolescence and into early adulthood, rather than reaching full maturity overnight.