What's the issue? So the answer was wrong. That is why you downvote the answer, say "Hey, I don't think you're considering these factors when answering," and then move on until the answer-giver replies to you.
That said, while I am not about to agree or disagree with the quoted text, I will ask what did they say that is offending? They aren't saying all men write about combat. They didn't say all young people do either. They didn't even say all young men do so. They said that topic (combat) is a typical focus of young, male writers. (For clarity, it reads more like they're noting a strong correlation, not necessarily a causation.)
Now, what's the distinction? Well, read the word choice.
Finally, combat is the typical focus of young male writers.
First, the wording does not exclude older writers nor does it exclude female writers. It doesn't even exclude young male writers from writing about other topics, as seen by the word "typical". The worst thing it does is it purports that young male writers will write about combat more frequently than other genres and it implies that writers in other demographics (whether older male writers or female writers of any age) wouldn't be so quick to write about combat.
Is this sexist? Maybe it shows a bit of the author of the answer's bias in regards to what people tend to write, sure... but the comment itself is presumptuous and lacks citation; not necessarily sexist. If it was based on data, then the same statement wouldn't be able to be seen as sexist at all since it's supported by data. Who knows, the claimed statement may or may not actually be correct. (I don't think it's right, but it could very well be correct. We just may never know since the user didn't provide citations to support their claim.) The person leaves enough elbow-room for exceptions, though, and while it may still be wrong, it doesn't claim to be exact, nor does it claim "Only young male writers can write about combat" or "Young male writers can only write about combat".
Personally, I think the quoted text isn't unfounded, I just think it lacks the support to push so assuredly. Don't get me wrong, even if correct, it is still wrong for a few reasons. The problem with it is the fact that "combat" is such a wide term that it can include romance stories like Inuyasha which include combat for the sake of combat (written by a woman) and it can include battle shounen like One Piece (which is being written by a 44 year old man... not old, but not young either). It can also include a slice of life series where the main character gets physically bullied and into fights. One-off instances are still combat. Then you have Detective Conan, a Mystery Thriller series that includes combat scenes as well, which was written by, at the series's start, a 30 year old man. Again, not young nor old. Now, if the quoted text backed up its claims with citations or narrowed down what it meant by combat: maybe it would be correct. It's just, right now, really wrong due to the lack of provided-reasoning and the moral implications of wording it in a way where correlation and causation can be potentially muddied.
Now, the question is this: Should a mod erase that text? No. Let the answer receive punishment by going through downvote hell. The answer isn't being supported, meaning the inferred bigotry isn't being praised. He didn't call anyone a slur, nor did he try to say it was a requirement for that to always and only be the case. No harm is being done, so just voice objections. Case closed.