You know, I've never for a minute considered that those books were marketed to anyone under the age of 20?
E.T. the Extra Terrestrial, came out in 1982. Not a single character seen playing the game in that film was in their 20s.
This is before your FLGS was common. The books were marketed to anyone who walked into a hobby shop or the sci-fi section of a bookstore, or some comic book shops in the 1970s and early 80s. That included a lot of adolescent and teenage geeky boys.