If I joined While I was 19 and Completed the program before I turned 21 What would happen? Would I continue being a CFI until I turn 21?
You could continue to instruct or look for another flying gig that isn’t Part 121 airline (corporate, charter, Part 91, Part 135 etc).
Make sure to check out the program prerequisites page. At 19, I find it hard to believe you’d meet those requirements. Even if you happen to, admission is incredibly competitive and you still might not get admitted.
My first question to you as well, do you have any flight experience?
If yes, check out the prerequisites I’ve linked below and maybe give it a shot.
If no, you need to do some research and take a few introductory lessons to see if this career is something you could see yourself doing the rest of your life (or a majority of it).