I was 11.
In the summer of 1986, and I was visiting my dad in New Mexico. He took me to see Red Sonja in theaters. A few weeks later we were in an Albuquerque mall and I wandered into a game store -- not a gaming store, this place specialized in high end chess sets, poker sets, and board games. But they had a display of the Mentzer Red Box D&D, and something about the Elmore cover art made me think of Red Sonja, and I practically threw a fit insisting my dad buy it.
It was a few months later, shortly after my 11th birthday, that I convinced two classmates, Jed and Josh, to play a game of D&D. They came over to my place and spent the whole weekend. My first dungeons consisted of a several rooms in linear sequence connected by hallways, with each room containing one monster and a treasure chest. There was a skeleton, an orc, and then a thoul that killed them both. We thought it was the coolest game ever and I must have run that dungeon, with no changes, at least 20 times in that weekend.