At the most basic level, type development is the process of gaining comfort and command of your preferred way of taking in information, and your preferred way of coming to conclusions. Developing a function involves consciously differentiating it from the others, exercising it, and becoming more skilled with it.
Jung believed that all the functions are largely unconscious and undeveloped in infants. As we grow and develop, the different functions develop. The timing of this development has been the subject of considerable study. It is generally believed that the dominant generally develops up to age 7, the auxiliary up to age 20, the tertiary in the 30s and 40s and the inferior or fourth function at midlife or later.
As you develop your type, the way you see the world and the way you behave tends to change and broaden. Comfort with your dominant and auxiliary functions forms the basis for much of your self-esteem.
If the use of your dominant and auxiliary functions is not supported by your environment, it will still press to reach the surface, like a beach ball held under water. When a function is never allowed to develop naturally, a person can experience stress and frustration.
As you develop your tertiary and least-preferred functions later in life, the range of behaviors available to you opens up even further. But the dominant and auxiliary functions will always be the core functions of your conscious personality.