Aging charaacters can create problems tho, plus one main character ages and they all have too. I don't mind them staying young for another 100 years, I realize that while I may have read comics since the late 80's-early 90's and went back and read backstories, that others are just now picking them up. If Spiderman suddenly aged tho and started to look a bit older, tried to do an acrobatic move and dislocated a hip, and started to get grey hair, then stood next to Iron Man in a cross over who looks like he was 30 it'd be too much of a suspension of belief. Second one main hero ages the others will all have to too, only way currently a hero in the mainstream aging without anyone caring is in an alternate future timeline.
To tell the truth I can't look at Spiderman as some cool almost 30 year old, with a super model wife and so on that I could hang out with. I subconciously see him being older, not 10 years older, but way older. I can't help it, growing up reading comics then seeing him the same age now, can't be helped. But once they actually make him age it's over more or less, he can't get younger unless he gets his hands on the vulture. I personally don't wanna read a Spiderman who's bones pop every time he throws a punch, and talks about arthritis every time he does a flip. And if he did start aging, and everyone else did to keep the continuity making sense, and sales dropped dramatically over the next 10 years, then they'd have to do a hero's reborn type stunt and reset the Marvel Universe, and you've seen how well that pans out.
As long as the characters progress on the inside, I could care less about their outward progression. Just as long as they don't do something stupid like Parker and MJ having a kid, then the kid (May) growing into a teen and being a Spidergirl in the 616 'verse, and SPidey looking close to her age still. Of course characters like Hulk, Wolverine, Sabertooth, and Deadpool I think won't have to worry about this

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Also agreed that simply putting a new guy in Iron Man's suit doesn't make him Tony Stark, only Rogers will be Capt. America in my eyes (even if he wasn't the first), the costume doesn't make the hero their character does.