I was around 11 when I discovered my INFJness, but I didn't think about it much at the time and I didn't really understand what it meant. When I started high school, I began learning more about MBTI and I began to understand what being an "INFJ" sort of meant (though I'm still figuring it out). The knowledge that I was an INFJ really helped me in social situations during high school and my first year of college because I understood that it was okay that I wasn't running around partying like everyone else. Now I'm trying to use it to help me figure out what kind of career path I should go down.
I found out I was an INFJ back when I joined this forum. It was such a relief to be able to be able to have words to explain myself better. roud: I was no longer a mistake, or someone somehow broken. I was just being me all along, and that it's ok to be me.
It's also been great to know more about the people around me too. It's helped with communication and understanding.