I was reading back through some CrossGen, some terrible Fantastic Four and a little bit of everything else, including golden age Captain Marvel and Superman & Batman. Yikes.
A couple of people around the place have, I think, really nailed it about what Marvel so ridiculously good compared to DC in the 1960s. The suggestion is that Marvel happened in real time, in contemporary settings (and a lot of crazy settings too), with the sense that a year of comic time was most often translating a year of real time, and when it wasn't, it caught up. The further suggestion one chap has made is that come the 1970s, Marvel lost its original thread, continuity became essential not as a simple byproduct of a real shared universe but as a means of referencing itself to confirm to readers that it really was plausible and happening in one Marvel cosmos. So in a somewhat subtle way, plausibility and ageing went together for Marvel- 16 year old Peter Parker became 26 year old Peter Parker, and the universe was believable and incredibly fun, with more of an old school newspaper strip feel - that a living breathing world was growing up. As Stan Lee himself noted and has referenced in interviews, the school students of 1961 became the college students and angsty young things of 1971, and kept right on reading Marvel, Viet Nam, culture wars and babies notwithstanding.
By the time that Marvel generation hit the 1980s, the die was cast- Marvel at that point had characters now old enough so that the "asian war" Tony Stark was captured in had to be changed, the Fantastic Four's Reed and Ben could no longer be WW2 veterans, and so on.
Understandably, but foolishly, Marvel blurred the age process and chose to start using the relative dating method that they still claim to use to this day- although in the last ten years their continuity has been entirely destroyed anyway and they clearly couldn't care less.
It would have been better for the heroes to retire or die, and be replaced by new iterations. It would have been cooler in the long run, and a lot fresher.
To the extent Marvel has been influenced by its imports from DC it has suffered horribly. Now there is a long list of Marvel characters who will never graduate, never fulfill themselves, never grow up in fact. Just like good old DC. Yikes, 2.