According to the theory on Type, you have one type that you’re born with and it stays the same your entire life. That’s not to say that you stay the same person. Everyone develops their lesser functions as they mature. Not everyone always gets the same results, but the theory remains. I think I got INTJ a few years back, but ISTJ definitely fits me better.
One thing to remember is that MBTI only tells you your preferences. It’s like being right or left handed. You can do things with both hands, but you prefer doing it with a certain hand, and therefore are more comfortable doing it with your dominant one and are likely to work with that hand. But, everyone works with both hands in day to day life. So the four letter description of your type just tells you your preferences. Nobody is always an extrovert or always an introvert, etc.
@Blueroses: According to a book I have, the American population is about equally divided between introverts and extrovert, but there seems to be a strong bias towards extraverts in our culture. (I seem to remember another book I read recently saying the ratio was 1:2, but I can’t remember which number refers to which preference)
@geeky_mama: Apparently as INTJ’s mature, they begin to develop Feeling more (their 3rd preference). Most people are mainly run by their lead and auxiliary functions until around their 30s when they begin to develop the third function, while the least function develops in the late forties or fifties if ever. It could just be that you’ve developed your Feeling.
For those who are wondering, these are the % of the American population estimated to be each type:
ESTJ (12–15%), ISTJ (7–10%), ESFJ (11–14%), ISFJ (7–10%), ESTP (6–8%), ISTP (4–7%), ESFP (8–10%), ISFP (5–7%), ENTJ (3–5%), INTJ (2–3%), ENTP (4–6%), INTP (3–4%), ENFJ (3–5%), INFJ (2–3%), ENFP (6–7%), INFP (3–4%)