I think, your type is determined by preferences. Which functions you prefer to use. This is related to, but not the same thing as the strength of your functions.
I agree with this. I typed myself as an INTJ because its functions are what I prefer but what I've noticed is that I have a very well developed Fi which sometimes causes me to be typed as an INFP. I used to get confused about what my real type is so I came up with the hypothesis that although a function may be prefered, it needs to be exercised for it to gain strength, much like a muscle. I think my Fi is strong because I often had to stand up to my senslessly abusive upbringings and understand the feelings behind the violent acts so I could attempt to stop them from happening again. (Which I was sucessful at doing by the way.) I mean I was actually beaten and belittled for accidentally breaking a cereal bowl. Talk about overreacting! However I identify much more with INTJ as I'm more organised and not very touchy-feely.
*Now of course this could be a bunch of BS I made up to rationalise my decision to type myself as an INTJ. But I still think it's a viable hypothesis at this time.
According to the science of the theory, your first function begins to develop @ age 6, the second @ age 12, the third @ age 25 and the fourth @ age 50.
As we are developing these functions, we are learning to prefer one over the other. One is becoming more comfortable, automatic and natural. This differentiation process is necessary for our psychological well-being. We have to have a preference or else we would be insane. According to what I've read, the only circumstance under which someone's type might change is if they suffer brain damage.
Well, and confusion also. It takes us a while to know ourselves.
"No ray of sunshine is ever lost, but the green which it awakes into existence needs time to sprout, and it is not always granted to the sower to see the harvest. All work that is worth anything is done in faith." - Albert Schweitzer
I think I've always been an INFP, but a while ago, when I would take [online] assessments, I would usually get INFP, but I would often get INFJ too. I didn't think this was accurate and didn't understand why I was getting those results until recently, I stopped getting INFJ results and started getting some INFP and some INTP. I think I (and I assume many other people do this too) go through phases where I exercise one of my non-dominant functions, yet still remaining the type I've always been. I think that accounts for some of personality change.