I wonder if the notion that computer wizards are kids isn't an artifact of the 80s and 90s. Back when computers were first reaching a wide audience, you often needed to do some work to manage to install games and have them run smoothly. The machines themselves were less reliable, but more simple than machines today. I don't mean the UI, which was not as welcoming as today's touchscreens, but the internal logic and organization wasn't as complex. So you had kids making the effort to learn these things their parents didn't really get.
Even if it was never the case that most kids were computer wizards, to many people, most computer wizards were kids. The way our minds work, that's just about the same thing.
No, the myth of the young computer wizard is alive and well, and a lot of kids believe the legends themselves.
I actually disagree with the notion that games are what separated the "wizards" from the mooks, and instead just the basics of what made computers interesting (Web Browser, Chat Room, simple applications) are what made the wizards of the 80's and 90's. Parents at the time were already familiar with games by and large via consoles - most of them probably had some version of Atari at least or knew someone who did, and the parents probably thought that all gaming was the same; just stick in the cartridge. I know that personally I had a heck of a time explaining why we needed computer upgrades for various games, or why DSL was essential for my Diablo and Warcraft experiences.
Often it's the simplest of tasks and knowledge, such as knowing how to quickly get accurate search results that aren't from Google/Amazon/Facebook or comfort with whatever office suite that elevates someone to the status of Wizard. A lot of folks have a surprisingly small comfort zone when it comes to computers and online, and being able to operate far outside of that tiny comfort zone is what gives them the impression of wizardry.
Even if it was never the case that most kids were computer wizards, to many people, most computer wizards were kids. The way our minds work, that's just about the same thing.