Human development is a complex process which continues up to the senile age. Lifelong people learn new skills, acquire new accommodation to the environment, use their previously-learned experience. Even after the birth, people already have certain physical or mental genetic experiences.
Except for biological stages of human development, there is a classification of the developmental psychology field which concerns mostly to the human ability to acquire certain experiences at a certain age. It is a six-stage adult development model synthesized from the views of Maslow and Kohlberg which characterizes how a person acquires new skills in the life independently of parents, teachers, and tutors.
At the first level, people are in their pre-adult development. They cannot take care of themselves, do not have a job and social skills. This stage characterizes people who are dependent on their caregivers and do not make plans for the future.
At the second level of development, adults aged 20 to 30 years old start their independent life. People are more concerned about their getting a job and a place in society. This is the time when young adults are anxious about their achievements which can be divided purely into victories and losses.
At the third stage which lasts up to 35 years of age, people get some rest from their professional ambition and get involved in the community. Usually, they raise children by this time and pay more interest to the social interaction.
The fourth stage is immanent to adults aged 35-45 who start to think more about their own identity. It is a time of introspection which may lead to a crisis of the middle-age.
At the last two stages of development adults actually find out their purpose in life and become comfortable with their achievements. Senior adults realize that their self-actualization came along with numerous life experiences. This is the time for people to be satisfied and enjoy life.