Is Shounen a genre?
Note that while the term "shōnen" tends to be used to refer to a few standard genres, it
isn't technically a genre as it literally refers to the target demographic (and in Japan, generally refers strictly to manga, rather than animenote Light Novels are often marked with a generic "male" or "female" demographic.).
What defines a shounen?
Literally meaning “few years,” “shonen” (少年) typically refers to
young boys under the age of fifteen. Thus, shonen anime and manga are aimed at that demographic. A lot of these anime and manga have a young male hero and are focused on action, adventure, and fighting.
What makes a shonen a shonen?
What makes an anime shonen is that
its primary target audience is preteen and teen boys. Shonen is more accurately called a demographic rather than a genre since content doesn't define it, instead it's defined by the age range and gender of the group it's primarily aimed at.