My first game (of AD&D 1E) was as a player, but technically it was with two experienced DMs. My two cousins, who lived an hour away from our house, were both experienced DMs and each ran his own campaign back home (with painted metal minis and everything). So one day, when visiting us, they brought their D&D books and some pre-generated PCs, I and my two younger brothers picked from the lot, and one of the cousins DMed while the other ran a shared PC between the two of them. They had each designed half of a dungeon, so when we crossed over to that point on the map they swapped, with the one who had designed that half taking over the DM role and the former DM running their shared PC.
That Christmas, I got the Dungeon Master's Guide for Christmas, one brother got the Player's handbook, and the other brother got the Monster Manual, and we were off. As the oldest, I became the DM, although my oldest younger brother tried his hand at it a few times. I've been DMing ever since, although in recent years I've also become a regular player, as my own younger son started up a series of campaigns (and then later one of our players decided to give it a try).