There are many different versions of the human stages of life, but most explanations include baby or infant, toddler, child, adolescent, adult, and mature adult. Each stage encompasses an age range. Some resources, such as the American Institute for Learning and Human Development, further divide these stages.
The human baby stage begins at birth and lasts until the age of 1. The next stage of human development, toddler, lasts from ages 1 to 3, and the child stage lasts from 4 to 11. Adolescence begins at 12 and continues to age 20, and adulthood spans from age 21 until 60, when it develops into mature adulthood, which ends in death. Sigmund Freud had his own version of the cycle: oral, anal, phallic and the full-genital. Carl Jung divided life into two stages; he described the rising sun as lasting through the late 30s and the setting sun as the gradual decline until death.
Biologically speaking, the human life cycle has many different stages, starting from conception and the development of the zygote into the fetus. More refined stages described by experts occur as a person ages and experiences physical and hormonal changes. Humans develop physically in every decade of life, including the zenith of the 20s, the decline of the 30s and the reproductive changes of women in their 50s, known as menopause.