What is a bigger population group people 40-70 or people 6 to 30?
Does every Grandma know about comic book characters? No but the vast majority kids to teenagers know about comics characters even if they don't follow it they are flood with ads and media about them not to mention many of them watch the cartoons when they where young kids.
If they are 600,000 grandmas who know comic characters,then they are 1 million kids to young adults who know about the Avengers in some form due advertising in media heavy era.
I would make a bet if we could do the polls,I willing to bet that Superman and Wonderwoman are more know through a wider age range aka 6 to 70 but iron man and green lantern are more known to people 6 to 30 and high percentage something like 80% to 90% and since that higher populate group more popular know about those characters.
Grandma knowing does not mean much if almost every kid in the US in elementary knows about it and there is Avenger cartoon that every kid would come across at some point or avenger commercial that every kid would have seen at some point