I know it's not very helpful, but girls do exactly the same thing, and complain that "boys smell" etc at that age. Perhaps you should just continue to treat your children equally sto set the good example.
I tell mine it is the sort of thing only very silly people believe. You could also give some extreme examples of a very cool girl (whether real or fictitious) and a boy that they think is silly - eg (if they watch Lazytown) 'That would mean Robbie Rotten was better than Stephanie, and that would be ridiculous, wouldn't it?!'
I am quite didactic about it and they love getting the answer right 'Now dd, who do you think is better, girls or boys?' 'Both of them!' which seems to work at this age (dd is 6 and ds1 is 4).
I don't think it's an innate age thing. I think it's to do with entering larger social groups (nursery, pre-school or school). I suspect that it's to do with looking for things that bond you to others in the group but have absolutely no evidence at all to support that
It's only anecdotal - but I notice it happens FAR less with dd's home ed from the start friends than her friends who went into school and suddenly started coming out with things like this. Though of course, it could actually be a completely different reason - like parental involvement or random other factor.
I dont know, DD (6) doesn't come out with derogatory things about boys as she is best friends with two boys at school and wants to be friends with everyone. I'd correct her if she did because I don't think its a nice thing to talk about other people like that. She used to be best friends with a little boy and she rushed up to him as usual one morning in the playground as usual only to have him push her away and tell her that he didnt play with girls as they were stupid. His mother didn't say anything but DD came back to me puzzled, hurt and with tears in her eyes.
She does get quite a bit of it at school though - from boys and girls who try and tell her what she can and can't do because of her gender. She told me that she says that 'well I;m a girl and I'm do it so it can't just be for boys can it?' but she shouldnt have to - and I suppose she is lucky that she is extremely confident with very thick skin. I have noticed that the two boys who do it the most and seem to be ringleaders have older brothers so perhaps that is where they are getting it from? I think that encouraging this sort of behaviour means its harder for children who don't fit into the pink/blue stereotypes to be themselves at school + encourages them to change themselves to fit in with their peers.
OP, I don't think you should worry - I think it's just a "Just William" phase they go through, although it can be hard not to panic and think "oh no he's gonna be a misogynist!"
My friends who have daughters assure me that girls can be super mean to boys.
I think you should challenge it gently but I don't think you should worry about it - unless of course he comes home with a book by Baron-Cohen or Pinker, then you should lock him in his room with no food!!!! lol.