After over 1300 (!) responses so far, I think the results are pretty clear: close to two-thirds of my regular readers entered the hobby in the first decades of its existence, which more or less corresponds with the era I call "the Golden Age of D&D." That's hardly a surprise given the focus of this blog, but it is interesting that the results I've gotten so far mirror those from an earlier poll on ENWorld from 2006, as pointed out by Delta.
It's interesting because ENWorld in 2006 was the center of the online D&D III universe. If you hung out there back then, chances are you liked and played 3e. Fourth edition hadn't been announced yet and the old school renaissance barely existed. Yet, even there, the vast majority (60.36%) of those who participated in the poll entered the hobby by 1984. Again, the results there are hardly scientific, but I don't think it's untrue to say that ENWorld's audience and mine aren't identical, even if there's undoubtedly overlap. Yet, the results of the two polls are fairly similar nonetheless.
What that suggests to me is two things. Firstly, D&D's status as the 800-lb. gorilla of the hobby was probably the result of a non-replicable fad. Barring some completely unforeseen turn of events, tabletop roleplaying is never again going to be as popular as it was in the first decade of its existence (and especially between the years 1979 and 1984). Secondly, the success of 3e was in large part driven by older gamers returning to the hobby after a long absence, not by the creation of hordes of new gamers. That's not to deny either the incredible feat that WotC accomplished in 2000 or that there were new gamers created in the wake of 3e. However, I think both those things tend to get exaggerated by gamers still running on the warm fuzzies Peter Adkison generated by saving Dungeons & Dragons from oblivion after the near-death of TSR in the late '90s.