Unfortunately, most of those superhero movies have adult content smushed into them. It rarely adds much to the story but does limit the lower age range of the potential audience. Seems like they're mainly aimed at horny teenagers and adults who still act like horny teens.
Those movies are incredibly violent from top to bottom and feature near constant scenes of characters engaging in activities that would get kids killed if emulated, or arrested if not killed. But a chaste kiss or profane language or mild inuendo is what makes them inappropriate for children? (The marvel moves in particular have been widely criticized for avoiding even implying that sex is happening anywhere, https://bookriot.com/why-are-mcu-superhero-movies-sexless/ )
Personally I don't think they'd be improved by adding sex, maybe improved by some better innuendo. I think movies in general WAY overuse sex scenes where its totally unnecessary or where some innuendo (or even just people heading to the bedroom) would suffice for plot purposes. It's not like movies usually acknowledge that people ever go to the bathroom, but you know if movie plots were real life half of them would have been tripped up because someone was in the toilet at the wrong time.
But when it comes to the marvel movies, I'm really at a loss at what adult content you think is making them unsuitable other than their fundamental violence.
