I'm glad to hear a post of mine has sparked at least an attempt at further conversation.
When WWII occurred 60 years ago is when they've become too old. Most of them were rid of in that Zero Hour crossover.
Or, when most of their original fans are dead.
Captain America had an excuse, at least. Frozen for a few decades. I think someone came up with the idea of super soldier serum and being frozen keeping him from aging (was it Neil Gaiman, perhaps?), so he's perfectly safe.
