Fantasy gaming is generally associated with a certain childhood sensibility. One firmly rooted in the innocence of youth, although important to retain going forward. But for those introduced to role-playing in the 1970s, gaming was actually an adult hobby that encouraged (and to some extent, demanded) that its younger aspirants measure up to adult standards of performance.
Indeed, the basic qualities of the earliest rules actively promoted these very "grown up" aspirations:
The earliest games had words. Lots and lots of words. And approximately none of them aimed at the young. These games were written by adults for their own amusement, and sometimes they addressed mature themes (one appendix to the Dungeon Master's Guide goes into detail about prostitution)...
But while it's always been tempting to imagine the "adult" as synonymous with the sexual, these early games were mature in their less salacious details. These were historical gamers first and foremost, and the earliest rules assumed knowledge of medieval armor and weaponry as well as culture, history, and religion.
Subject matter aside, the early games were simply written to an adult level of reading comprehension. And young gamers who wanted to participate had to do so on adult terms.
Additionally, illustrations were not only amateur, but also less frequent, meaning that readers were confronted with - well - lots of written words. This gave the pictures decidedly more impact while simultaneously leaving more to the imagination, which was a good fit with the written words, providing detail while inviting readers to fill in their (ever-present) blanks.
Now, imagine an 11 year old kid back in 1978. He (and that was obviously yours truly) joined a group of older players and had to make sense of demanding (read: adult) rulebooks. But this kid wasn't deterred. Rather, he was EXCITED and found himself WANTING to rise up to the next level and master it on ADULT terms.
Later, by the early 80s, D&D was marketed to younger readers, with its writing retooled accordingly. This wasn't really that bad, although it made it increasingly less necessary for young readers to measure up, as the writing met them halfway! The new artwork was likewise greatly improved, not to mention more frequent...
More and better pictures plus fewer words written to a younger audience shifted this dynamic greatly, although role-playing remained a decidedly intellectual (and complex) pursuit favored by intelligent people, and I won't denigrate that...
Nonetheless, the waning historical references coupled with increasing details pre-imagined by means of superior illustrations and easier-to-read explanations saw the end to an era.
Back in 1978, amateur rulebooks with primitive illustrations surrounded by many words written by (college-educated) historical war-gamers encouraged this blogger to ASPIRE towards adulthood, ironically, through an imaginary world of magic, monsters, and the stuff of childhood fantasy. It completed his education and even joined him in war, noting that all those imaginary combats on paper taught strategy and problem-solving; adult life in fantasy form!