The psychology of life cycle is a branch of psychology that tries to relate the place where an individual is in the course of his/her life with the kind of issues that the person is facing and with the kind of resources s/he will have available to face those issues. And, eventually, the kind of disturbance s/he could develop in case s/he fails to cope successfully with those issues.
From a theoretical point of view, the concept of stages of life comes from the thinking of developmental scientists.
This concept not only involves the idea that there are different phases in life, but emphasizes the belief that any phase builds upon the previous ones.
In the same way a boy can't learn how to divide or multiply without first knowing how to add or subtract, the fact of forming adult relationships outside of the family of origin is problematic for a person who never had acceptable relationships at home.
When understood in this manner, the concept of stages of life is a very powerful one.
For instance, this means that, in diagnosing a patient's disturbance, the life cycle psychologist will attempt to determine at what stage the individual failed to meet the task of that phase of life. The person's poor performance in the stages that follow can then be understood in terms of the lack of the required preparation for that stage.
Thus life cycle psychology is in favour of using therapy as an attempt to correct the deficits created by the unsuccessful completion of a previous stage in life.