"Boomer shooter" is a genre of video game. As far as I know, the term is unrelated to "Baby boomer," the name of the generation.

Edit: Okay apparently it is related. Some debate exists over the etymology. I imagined it was from "boomstick" (a gun), but some people are are claiming it's from the dusk development team and a reference to a meme I'd never heard of about calling people boomers who aren't, or maybe it's from earlier and nobody can trace the etymology at all.