Theoritical basis from historical angle

Sigmund Freud divided people’s (psychosexual) development into 5 stages: Oral, Anal, Phallic, Latency and Genital. According to his psychosexual theory, people’s development has been mature till adult, and personality has been completely formed. It seems to mean people’s development would stop at the early stage of adult. In contrast, Erik Erickson’s proposed a theory of development that continues throughout the life span. His theory states that there are universal life stages and that a specific psychosocial dilemma occurs at each phase of development. These problems (crises) must be resolved before an individual can move to the next developmental stage. Erikson's theory has been credited for accounting for continuity and changes in personality development. In Erickson’s theory, generativity versus stagnation also coincides with the idea of a midlife crisis. Erikson believed that in this stage adults begin to understand the pressure of being committed to improving the lives of generations to come. In this stage a person realizes the inevitability of mortality and that they will not be around forever and the virtue of this stage is creating a better world for future generations in order for the human race to grow. Stagnation is the lack of psychological movement or growth. Instead of helping the community a person is barely able to help their own family. Those who experience stagnation do not invest in the growth of themselves or others. This is relatable to midlife crisis because a person becomes aware of the time they have left to live and decide how they want to spend that time..

Jung tended to see midlife crisis as a spiritual crisis—focusing on inner transformation and introspective depth. Therefore Jung relied more on a number of his dreams to explore his midlife crisis. In his Memories, Dreams, Reflections of dreams and waking visions, powerful transformative images sequently presented five potentially major themes of male midlife transformation:
(1)The demise or diminishment at midlife of the youthful and blindly self-assertive heroic attitude;
(2)The problematic coming to consciousness of the shadow (the repressed contents of the personal unconscious).
(3)The resurgence of a man’s feminine side (his anima);
(4)The appearance of a midlife mentor; (5)The acquisition of a new sense of identity.

References
Erikson, E. H. (1963). Childhood and society (2nd ed.). New York, NY: Norton
Frankl, V. (1967). Psychotherapy and Existentialism. New York, NY: Simon and Schuster.
Huyck, M. (2002). Why Are Some Married Men Vulnerable at Midlife? Washington, DC: American Psychological Association.
Jung, C. G. (1923). Psychological types. London, UK: Pantheon Books.
Vaillant, G. E. (1977). Adaptation to life. Boston, MA: Little, Brown.