Per quick Googling: In 1974, Gary Gygax was 36 and Dave Arneson was 27; M.A.R. Barker (Tekumel) was 44, Dave Hargrave (Arduin) 28, Greg Stafford (Glorantha) 26; C&S co-authors Wilf Backhaus and Ed Simbalist were 28 and 31 repectively; Ken St Andre (T&T) was 27, Marc Miller (Traveller) 27, Jim Ward (MA, GW) 23. Rob Kuntz (Greyhawk co-DM) was 19.
I have seen the suggestion that Ed Greenwood was but 15 in that year, which would make him only 7 when (in 1966) he allegedly began writing stories set in the Forgotten Realms, and 19 when by his account he "really embraced [D&D] and started playing in 1978."
Considering that all these early 'pros' started out as hobbyists, that the business was still largely on that semi-'amateur' level (with producers being also consumers), I think this is probably not a terribly unrepresentative sample of the demographic that was getting turned on to D&D in the first few years after publication.
The average age in that set (including 19 for Greenwood) is about 28.
D&D was advertised initially in venues catering to the historical-wargame hobby, especially the branch employing miniature figurines and model terrain in place of cardboard counters and paper maps. (TSR originally stood for Tactical Studies Rules, and The Dragon's predecessor was The Strategic Review, companion to Little Wars.)
That has always, I think, skewed a bit older than much with which D&D has since become strongly associated.
I don't think there was very much of an advertising and distribution effort beyond that fairly obscure hobby/industry until 1977. The scene until then seems to have grown mainly by word of mouth, and that usually traced back by some route to Midwestern wargamers -- so the vectors were notably less diverse than later.
In the 1980s, a wide variety of retailers carried Advanced books and Basic sets, modules and magazines and often miniatures. Especially after the Dallas Egbert case (summer of '79), there was a lot of free publicity (albeit often scare-mongering) in the mainstream media. RPGs, especially D&D, started to turn up in features such as the Games 100. Programs for personal computers, including cartridges for consoles, brought D&D-derived concepts to a wide audience even when they did not refer to the game by name (which articles in computing magazines sometimes did).
Basically, I think it was the 'breakout' from the original hobby market via word of mouth (and 'pirate' copies, as demand exceeded legal supply) that made it feasible for TSR to mount a wider marketing program. The older-skewing bases were simply too small, too quickly saturated, whereas association with enthusiasms more common among kids -- toys, comic books, video games, cartoons, even fantasy-themed board games such as TSR's own Dungeon! -- "snowballed" to attract a larger and larger market.