[Athenian200]

Well, there's really no similarity between IJ and EP. People only notice the Extraverted function in real life.

That's the period of meaningful development. Before that period, most people make primitive, simplistic judgments or perceptions. So yes, you always have both, but you don't really start using a process consciously and deliberately until it starts to develop. You do notice that most people don't seem as able to control their actions, plan, or perceive situations clearly before the age of about 12, right? In fact, in one culture (can't remember which), 12 is the "age of accountability," and before that you couldn't be held accountable for a crime because you aren't considered "aware" enough to know right from wrong.

So from the moment you start interacting with the world, you're able to use all functions in a primitive, basic way. Developing them past that basic point is what's referred to. For instance, even an INFP has enough "T" to learn how to use a doorknob, or seek and fetch an object for another person from a fairly early age, but they would struggle with more complex T-oriented tasks. And even an ESTJ experiences emotional discomfort if someone is cruel to them, but they may not be as able to express or deal with their discomfort, and would tend just react to everything with either anger or repression until reaching a higher level of development.

Basically, the functional order isn't as meaningful as you'd think... all that's known of Ni and Si is that they lend themselves to J-oriented behaviors. IJ's usually seem more "J" in keeping track of things they need to do and having long memories, EJ's seem more "J" in pushing their agendas on others and getting more things done. But both types tend to do both these things to some degree.

And all that's known of Ti and Fi is that they lend themselves to P-oriented behaviors, but I can't provide as much on that.