In conversation with a fellow science fiction writer a few days ago (Nov. 2020) I realized I actually had my debut as a science fiction writer at age 11-12-13:
A tabloid, most likely BT or Ekstrabladet, made a competition for something extraordinary (in the realms of the fantastic, as I recall) towards winning movie tickets, so I wrote a short story about seeing an UFO, which won me the tickets and the story printed in the paper.