Gen X is the cut off point, my parents were young in the 80s so they basically know what a video game is, they've even played one or two before, however it was way more of a niche interest like a lot of 'nerd' culture was back then. It's the statistically average age of the audience but with video games, the technology is still within some people's lifetimes, so the older demographics are exponentially less likely whereas the younger demographics don't really have any limitations except flimsy age restrictions. In terms of demographics, 25-39 'millennial' demographics are who Fallout is marketed to.
