I don’t know. I have vague recollections of running (or trying to run) B2 Keep on the Borderlands in 5th or 6th grade (so … 1983? 1984?). I ran games sporadically throughout middle school and high school, but most of what I did was world-building that never turned into an actual game. It wasn’t until I went to Lock Haven University in 1991 that I really started game mastering regularly, with a weekly campaign that lasted throughout college. That’s also when I started running games at conventions (and heck, started running conventions).
I ran my first proper campaign in Greyhawk, having acquired the World of Greyhawk and City of Greyhawk boxed sets at some point during the 1980s and From the Ashes boxed set in 1992. I used the Advanced Dungeons & Dragons 2nd Edition rules (which were still relatively new, having been released in 1989). After college, that campaign would go on to become the foundation of my weekly gaming group’s longest running campaign, the Blackrazor Guild.