"The term boomer shooter has a rather nebulous origin, and it likely started as a joke. Online pedants often point out that the original first-person shooters were developed by Gen-Xers like John Carmack (born 1970) and John Romero (born 1967), not Baby Boomers. However, "boomer shooter" uses the slang version of the term boomer, as a stand-in for any older person who is closed-minded and out of touch—so please, direct those complaints elsewhere."

Source: https://store.epicgames.com/en-US/news/a-beginner-s-guide-to-boomer-shooters-and-how-they-inspire-new-school-fpses-like-witchfire