This is quite a complex matter, but broadly speaking there are only 16 innate possibilities for type development in an individual person. The dominant is meant to develop first, and for most people this happens at a very early age, possibly pre-natal, and, I believe, has a genetic component. Developing a function involves setting up complex neural networks in the brain and nervous system. This is achieved by forming lots of connections between the neurons related to that function. It’s a bit of a chicken and egg situation: using a function causes more neural connections to form, and functions with many connections are easier and faster to access and require much less energy (derived from glucose) to operate, so are more likely to be frequently used, causing more connections to form, and so on.
The development of brain in the first years of life is at least as spectacular and rapid as the physical growth and development. This link provides an excellent summary of the process.
In the first few years of life, more than 1 million new neural connections are formed every second. After this period of rapid proliferation, connections are reduced through a process called pruning, so that brain circuits become more efficient. Sensory pathways like those for basic vision and hearing are the first to develop, followed by early language skills and higher cognitive functions. Connections proliferate and prune in a prescribed order, with later, more complex brain circuits built upon earlier, simpler circuits.
The brain is most flexible, or “plastic,” early in life to accommodate a wide range of environments and interactions, but as the maturing brain becomes more specialized to assume more complex functions, it is less capable of reorganizing and adapting to new or unexpected challenges.
For example, by the first year, the parts of the brain that differentiate sound are becoming specialized to the language the baby has been exposed to; at the same time, the brain is already starting to lose the ability to recognize different sounds found in other languages. Although the “windows” for language learning and other skills remain open, these brain circuits become increasingly difficult to alter over time.
In type terms, this early specialisation is intimately associated with the development of the dominant function. Less preferred functions will fall away as part of the neural pruning process, and once this has occurred, those functions will forever be harder and more energy-draining to both use and develop.
The functions that require similar neural networks to the dominant will be the first to fall away in the pruning process, for example, Ni and Si both require access to the internal visualisation areas of the brain. Developing Si means that Ni is locked out from these area, as the circuitry has been specialised to examine internal objects at close range to see all the small details with clarity. The opposite is true for Ni, which has a long-range vision and scope in the internal world, as if one were scanning the horizon or looking down from a great height. Since you cannot look at a near object and a far object at the same time, when Si is operating, it is impossible to access Ni, and vice versa.
The auxiliary function has a bit more leeway than the dominant; for each dominant there are two possible auxiliaries that can be developed, so nurture plays more of a role here. The auxiliary and the dominant draw upon different, but complementary neural networks, one is always a perceiving function and the other a judging function. This means a developed dominant does not hinder the operation of the auxiliary to a great degree. But once the auxiliary is developed, it too hinders the development of related functions, Fe precludes Te, for example.
The tertiary and inferior partially overlap with the neural networks used by the auxiliary and dominant, but because they operate in the opposite attitude (extraversion/introversion) they also partially draw upon different, complementary networks. This means they can still be developed to a certain degree. Because the auxiliary is less developed than the dominant, there are more undeveloped neural networks available to the developing tertiary function than there are for the inferior. This explains why there is more scope for the tertiary to develop and be available to the conscious mind than there is for the inferior, which must make do with the slim pickings left by the other three functions.
And once all four primary functions have been established, there is precious little neural capacity left for the remaining four functions, forever condemning them to the shadows of the unconscious mind.
All this assumes normal type development, in other words the child’s inherent typological nature is fully supported by the environment. In reality, of course, this is rarely the case; even the most supportive of families will have their type biases and will inflict these upon their children to some degree. This is not necessarily a bad thing, as it can encourage the development of less preferred functions before they are permanently locked out by the neural pruning process.
If it goes too far, however, then it stunts and inhibits a child’s innate type development, creating differing degrees of psychological and emotional distress and trauma. In extreme cases it tips over into serious abuse, and is likely to lead to long-lasting mental and emotional illness in the child with all of the attendant suffering and pain.
Abuse of this nature can skew and distort the natural type progression, creating anomalies which fall outside the 16 healthy types. This can be extremely unhealthy and pathological for both the individual and society. Just think of the examples from daily life: the loner/misfit who one day goes on a murder/suicide rampage with a lethal weapon could be attributed to an extreme lack of healthy extraverted development.
Lack of healthy introversion can lead to low self-esteem and unhealthy dependance on others for direction. This is perhaps more commonly seen in women, who are more likely than men to remain in an abusive or toxic marriage.
See point 5 in the linked article for a discussion on how toxic stress damages developing brain architecture, which can lead to lifelong problems in learning, behavior, and physical and mental health.