This is because, during each stage in your lifespan, it is possible to have a psychosocial crisis that could have a negative outcome for your personality development.
A psychosocial crisis involves a conflict between your psychological needs (psycho) with the needs of society (social). At each stage, there is a crisis or a task that you need to resolve.
To see where your developmental impasses are, I welcome you to take a look at this article on my Depth Therapy Blog that details all the stages of human development.
When you can identify an unhealed personality part's age, you will better be able to see what kind of emotional healing work you need to do in order to progress in your life.