"Summer 1974 - Glenn had played a game while visiting relatives, we didn't have the rules, so we sorta did stuff based on what he remembered."Summer 1975 - Somehow we were able to bootleg a copy of Greyhawk, just after it came out (we never did find anywhere to buy it). This gave us enough to play on. That summer we did little else.
"Jan. 1976 - We finally found a place where we could buy The D&D (white) Boxed Set with the 'soft' dice. I have to assume it was a second printing, since web sources claim the original 1974 printing was only 1000 copies. Finally figured out all the stuff we'd been doing 'wrong'.
"Having established a source, Blackmoor, Eldritch etc. was purchased as soon as it was released.
"August 1978 - the day before I left for CMU I spent the morning waiting at Dream Wizards for the truck carrying my copy of the DMG to arrive.
"Therefore at least 25 years, and I'd say 26.
"Although - I could make a case for much earlier than that. When we got tired of playing army/cowboys & indians we did move into stuff that *was* role-playing: Star Trek, Legion of Superheros. It was a live action free form style of gaming, but the role playing was present; the person who got to be Spock today, was Spock and played different than the day it was their turn to be "Bones". For the Legion stuff, we made the paper bag vest style costumes for most of the Roll Call and would do costumes changes as the situation called for different powers. This complexity clearly indicates there were definitely "rules" involved (although unwritten), and distinct roles. I'd date this as no later than 1972 since we were still living on Irving street. The Star Trek trope involved though says probably no earlier than 1967. Prior to that we were generic army men/indians/good guys/bad guys - no characterization, so while I know we played these games earlier, I wouldn't label them role playing.
"So .... 32 years would be a defensible answer."