I was inspired to write this little explanation of how I imagine type development occurs in children by the thread about type impersonation. I was originally responding to that thread but then my thoughts grew loose and baggy and I realized I really ought to give them their own thread, both to allow them to stimulate (hopefully) greater discussion, and to avoid threadjacking the type development bit. Anyway, here's my thoughts as to how very young children develop type:
Hmmm... well I think of myself as group A and voted that way but now i wonder if my theory of type development corresponds all that closely to group A.
The way I think about it is that babies find that, say, manipulation of and focus on the tangible (sensible) elements of their environment brings them pleasure and fulfills their wants/needs/desires. Since it tends to work, they tend to keep doing it. They tend to get better at it. This preference gradually becomes ingrained in their brains by some mechanism (e.g., increased density of neurons), so that it is eventually (at some uncertain point that's probably different for every individual) impossible to change. This child develops into an Se-ego, eventually picks a secondary function by a similar mechanism, and these two functions (primary and secondary) dictate the remaining functions by the laws of model A, and that order of functions is what we know of as type. (And then type tends to produce certain habits of mind which are what we most often see in the personality of the individual of a given type, especially in the case of irrational functions).
Thus the child develops: increased competency, a preference for a certain style of interacting with the world (that is, a sort of "this is what I try first" approach to things), and a sense of pleasure on performing the act (this degree of pleasure is also probably ingrained by a neurological mechanism, either positive---this action also produces some sort of memory-activity, subconsciously reminding the individual of prior success---or negative---this action requires less effort for the brain, less effort/confusion produces more good feelings (the opposite of the discomfort created by the neurological confusion in the minds of the listeners at the first performance of "The Rite of Spring.")).
Now, this mechanism of development, relying as it does upon the successful use of information aspects by children, has two components, an external one and an internal one. The internal component splits again into two components: natural competency and natural preference.
Natural Competency: The child's natural competency at a particular mode of interacting with the world (i.e., information element/aspect) is likely to influence her rate of success at using that information. Theoretically, if the child had a weak sense of sight, smell, taste, touch, and hearing, he or she would probably not achieve anything desired by focusing on and complexly manipulating their sensory environment, and would not develop into an Se-ego. (Note that this does not preclude the possibility of, say, a blind-from-birth Se-ego---who could have relied heavily on the other heightened senses---but does imply that we should see fewer Se-egos among the seriously-disabled-from-birth than among others.)
Natural Preference: I do not reject the possibility that individuals have a natural preference for certain information aspects (although we should note that they do not, strictly speaking, have a preference for an information aspect as such but rather for an aspect of reality---invisible connections between things, emotional expressions, sequential and organized logic, etc.---which overlaps with the information aspect).* In any case, if this natural preference exists, it stands to reason both that the child would attempt to achieve goals in the preferred way more often, increasing their success rate in two ways: 1) assuming that children are satisfied in most of their important aims, which I believe they are, more attempts = more success, and 2) increasing competency.
The second component of my type development method is the external component. Obviously, a child's success or failure at any endeavor depends less on their skill than on their environment. This is where the accidents of fate and chance come in. A child with a natural preference that might lead towards an extraverted thinking type could experience some sort of crushing defeat in a situation wherein they attempted to rely on the precursor skills of an extraverted thinking type (e.g., it is generally relied-upon, a generally established fact, that mother provides nourishment but suddenly for whatever reason, the child expects nourishment from the mother and does not receive it for quite a long time). This external factor might cause them to eventually develop into another type (with a resulting degree of neuroticism or something like that depending on the strength of the natural-preference-factor).
Obviously, all of this occurs very early in life. I imagine that primary function is set as early as six months, and probably no later than two years old, while the secondary may be unchangeable somewhere between three and five years old. (And again, the rest of the functions are determined by these two).
*In fact, I consider it entirely possible that this natural preference both exists and is the driving force of type development. In any case, I would imagine that the situation is such that with the natural preference, the child gets an equal or greater amount of pleasure from how the action is done (IA used) as from the success or failure of the act. This would be an extra source of pleasure not accounted for in my original schematic of type development, and could very well be the motor that runs the whole ship, as it were.
So yeah, what do you think?