I have things that I can confidently call conworlds going back to my middle school days in the late 90s.
My current Lonely Galaxy setting is a rework of an earlier project I called variously The Last Grand Adventure or En after the in-universe name of the world. It has roots going back to my freshman year of college in the early 2000s.
I conworld mainly as a means of escapism. My current conworld was born out of a time of extreme stress. I couldn’t handle the real world, so I retreated into my own. Perhaps not the most healthy way to cope, but eh.
My sci-fi setting can trace its roots to sketches I used to do years ago in class. (Some of those sketches have been updated into 3D models, and yet more are still ‘canon’ to the present iteration of the setting - if not necessarily released.) I found I could scratch out what was on the board and draw sci-fi equipment at the same time, so I started dragging a sketchbook around to class with me.
But the nature of the setting has radically shifted. It’s gone from a “20 minutes into the future” series featuring a stateless army dedicated to preserving world peace, to a ~200ish years in the future setting featuring conflict between various interstellar states following a bitter war across the solar system and an examination of the ideological differences driving them.
Ultimately, though, it’s just an excuse to make up lots of near sci-fi equipment.