Comic book movies appeal to a wide audience
The comics that most comic book movies today are based on came out in the 1960s, '70s, and '80s. Lots of adults with disposable income were raised on these heroes and their stories. Younger demographics enjoy the novelty of these stories. This allows comic book movies to appeal to a wide audience.
Comic book movies appeal to young and old audiences alike. They are able to do this by using nostalgia to appeal to the older demographic, who grew up with these stories and characters. In order to appeal to young demographics, they simply tell the story. The comic book industry crashed in the late 1900s, which means that most kids born after late 1993 didn't grow up reading comics. By simply telling a compelling story, comic book movies are able to appeal to a younger audience with what they perceive to be imaginative and original storytelling. These same storytelling techniques have been around for decades though, in the panels of comic books. Older generations encountered these compelling, interconnected stories as children. Because they remember their childhood experiences with this medium fondly, older demographics also flock to the theaters to see comic book films. Thus, comic book movies are popular because they utilize nostalgia and novelty simultaneously to appeal to a wide audience.
While comic book movies do appeal to adults, this isn't the only factor for their popularity. Further, although the comic book industry did collapse in the 90s, it didn't die, meaning some kids still grew up with comics. Comic book movies appeal to comic book fans in general, not just the older and younger demographics.