John David Washington has already had to defend the age gap with Zendaya for Malcolm & Marie and the movie isn't even out yet. He points out that she has been in show business much longer than him. Also, the movie was her idea, so if anything, she picked him. The story is about a director and his aspiring actress girlfriend, so it's not like the age difference is unrealistic. It apparently gets addressed in the movie.
Another thing is that Zendaya got her big break as a Disney kid and still plays a teenager on a TV show now, so some people can't deal with her playing an adult (I guess this is why there wasn't a huge objection to Zendaya/Zac Efron in The Greatest Showman, because he's also an ex-Disney kid). People say she looks like a child next to JDW, which I think is ridiculous. The near-ubiquity of twentysomethings playing teenagers in TV shows/movies for decades has skewed ideas of how teenagers actually look in real life. It's also funny to see the Zendaya fans saying, "No one complained about Jennifer Lawrence in Silver Linings Playbook!" I guess some of them really are too young to remember the years of people saying JLaw was getting roles that should have gone to older women, bless...
I get that large age gaps in films, if they only go in one direction, can be indicative of a system that easily discards women while endlessly propping up men. OTOH, adults in different age ranges can have a good rapport and even find love, if they want. Maybe they'll work as an onscreen couple; maybe they won't.
Frankly, more annoying (to me, anyway) is the subset of social media which, I swear, is "disturbed" and "uncomfortable" with any age gap greater than 2-3 years. I mean, good for you that you personally never ever want to date anyone that you couldn't have theoretically attended middle school with, but that does not mean every relationship with an age gap is grooming or has a troubling power imbalance. Do you want to raise the age of adulthood to 25 or what?
And IMO not enough people entertain the idea that maybe it's the men who are too old for some of these roles. For the last decade or two, Hollywood has treated men like they are boys until they are 30, so no wonder onscreen (and offscreen) pairings work out how they do. Even in the Golden Age, you would get the 50+ men with a woman in her 20s, but they still let guys in their 20s play actual adults in mainstream, big budget movies.