Early adulthood, middle adulthood, and late adulthood are the three main stages of physical, emotional, and psychological development. In terms of physical development, early adulthood is the least dramatic. This stage takes place from approximately 20 to 35 years of age. Piaget believed that the formal operational stage (ages 11 to 15) is the highest stage of thinking. ▫ Adults gain knowledge, but ways of thinking are the same as.