I'm 46 and started playing/DMing with the Basic/Expert (red/blue) boxes around 1985, and I'd say I've DMed the most under that system in the mid-late 1980s, and, over the last decade, using Pathfinder 1.0.
I have also DMed some AD&D 1 (but not a whole lot), a bit more AD&D 2e and D&D 3.5, then...