They realized that long, long before. It's just that it was easier to maintain the illusion that characters were aging in semi-real time for the first twenty years or so.
Once you get thirty years or more into these character's lives and they're still hovering around the same age range, the illusion is inevitably broken and you just have to accept that these characters don't age in "Marvel Time" so much as, after a certain point, they just don't age at all.