I'm starting a new short story, and I think this one is going to be about a non-popper who becomes a popper. Not venturing too far out of my own experience yet, but give me time!
My conversion experiences happened in my late teens, only about five years after my first sexual experience with balloons. It kind of started with the TV show Wonderama (if you're too young to remember Wonderama, try to find reference to the "Balloon Bottom Relay" on the Internet; you will not find any footage, which is just as well because all the participants were underage. But then, at the time, so was I).
Then a girl that I lusted over told me that she like to pop balloons by sitting on them. At first I was repelled by the thought, but since I wasn't repelled by the girl I started to have fantasies about her butt in such hard contact with a balloon, and my fetish took a very different direction after that.
But I think I'm leaning toward an adult conversion in this next story. I want to make sure I set up the character's fetish in such a way that the conversion is believable even after all his or her time being a non-popper (haven't decided if the main character is male or female yet).
That will probably involve a lot of time trying to recapture the way I felt before I was excited by balloons popping. I don't think that will be too hard; I still enjoy non-popping play, and I like to keep balloons around for a long time, as I've mentioned here before.
As to the actual conversion event, that will probably involve a lot of fantasizing. It's a tough job, but it's got to be done.