Yeah it reads more like an intro to FPS play for anyone who is new rather than something age specific.
I'm 49 and have been playing multiplayer online FPS games since Quake. I currently play PvP in Destiny 2 which is one of the fastest movement FPS games currently played and I'm ranked pretty highly in Elo across most of the playlists in the game:
https://destinytracker.com/destiny-2/profile/bungie/46116860...
My reflexes haven't declined to any great degree. I'm sure if I measured them yearly since my early 20s there would be a measured decline, but it would be something on the order of handfuls of milliseconds, not a giant change despite the fact that I'll be half a century old next October.