I've gone through periods when I was more extroverted than others, but I've always been an introvert.
My parents told me that on my first birthday, I got freaked out about how many people were coming to my party. I always was extremely shy around people, and liked to spend hours focused on something that caught my interest. I was very type 5 until I was around 5 years old; isolating myself from others learning everything I could. Instead of talking to people, I'd spend time in my imagination or reading a book or drawing. When I was about 6, though, things changed drastically. I suddenly felt the need to crawl out of my shell and show people how much I cared about them. I'd play all sorts of games, and started becoming quite loud at times, making up games and sharing ideas like crazy, but I'd always go back to hours of contemplative silence.
Then I was bullied a lot in public school, and became very much a type 4 introvert. I would often be of the mindset that nobody understandood me, and became quite emotional, much to my own embarassment. I seemed somewhat extroverted, because I wanted to make people like me, but I still had a thirst for quiet and privacy. This led to a time of extreme introversion, and I didn't become a stable 4w.5 INFJ introvert until around grade 10.
So, I think you're pretty much born one way or another, but events in your life cause you to adapt toward the opposite temporarily, especially as a child who is learning about how they fit into the social world.