The stages of life have been defined by cultures and religions in many ways. In ancient Greece, the human life cycle was mapped in seven-year periods. Today, most people recognize the human life cycle as having four or five distinct stages shared by all humans. Human life can be explained more concretely by looking at these life stages.
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Birth takes place between fertilization and 40 weeks following fertilization, at which point the baby is fully formed and ready to exit the mother's uterus and enter the world. During the birth stage, the baby begins as a single cell, which proceeds to multiply into many cells that form the body parts and organs of new human life.
Infancy is categorized as lasting from birth through the first year of life. At this point, the baby exits the mother's uterus. It can breath, its heart can beat and its organs can operate efficiently without the assistance of the mother. However, the infant is completely dependent upon its parents or caretakers for survival.
Childhood takes place between ages 1 to 10. The first two years of childhood, the child is called a toddler. During this time, the child learns how to walk, talk and be more self-sufficient. These skills continue to expand during the remainder of childhood, and socialization takes place. Childhood is the building blocks upon which adolescence and, later, adulthood will be built, and the child is susceptible during this time to learned habits and behaviors.
Adolescence takes place between ages 12 and 18 and is a critical turning point because it is when puberty takes place. Boys' voices change and girls get their periods and both sexes become more sexually aware beings. As such, they begin to separate more from the parents and become more independent.
Adulthood is the longest stage and normally lasts from age 18 through old age. While there might be smaller psychological or culturally defined stages, adulthood is when human beings are fully grown and must provide entirely for themselves using the skills they learned throughout the first life stages. This is also a significant time because it is when the life cycle is initiated again by the conception and birth of the adult's own children. At the end of the adult life cycle, the body begins to deteriorate and the life cycle eventually ends in death.