All living things go through four stages called a life cycle. The shortest stage is infancy, lasting from birth to 18 months. You spend most of this time sleeping because that’s when you grow. The next stage, childhood, lasts until age 12. You reach half your adult height at five years of age. The period between childhood and adulthood, called adolescence, begins at puberty. You undergo both physical and emotional changes. Between the ages of 18 and 22, your bones stop growing. As women age, they go through menopause and are unable to have children. Some older people react more slowly and experience memory loss.