|The modern (i.e. post-FF#1) Marvel Universe began in the 1960s but by the year 2000 they had barely aged at all. Considering comics come out once a month and generally document a day or two's worth of actions, this hardly seems like a problem. Consider Spider-Man: he first appeared in 1962. At the time he was 15 years old. If we assume conservatively that each comic book documents a week's worth of activity (it's probably much less - consider cliff-hanger endings where several issues (i.e., months) actually cover the events of one day), by 2000, we had seen (2000-1962) * 12 weeks = 456 weeks of activity. That's about 9 years, putting Peter Parker in 2000 at 24. He's generally depicted today as being around 30. So the fact that characters that were introduced in the 60s are still relatively young by 2000 isn't really an issue. The problem is that this 9 year (or 15 year, or 7 year - depending on how you calculate it) period in which all modern Marvel activity occurs means that, looking back from the year 2000, it must not have started until 1991 (and it will have to be a sliding timescale. In the year 2020, the Marvel Universe can't have started until 2007).
There are a lot of problems with this way of looking at things. For one thing, a lot of characters have origins tied in to real world events that take place before 1990. The Black Widow and a lot of other characters have roots in the Soviet Union. The Punisher's personality in part was formed due to his experience in the Vietnam war. Writers have been addressing these concerns by subtly altering the origins of these characters - replacing Vietnam with an unnamed military conflict, for example. To my way of thinking this is done at major detriment to the characters - the Punisher, was, at least in part, intended as something of a metaphor for the shattering experiences soldiers, and the country in general, faced because of the Vietnam War specifically. To take away that aspect of the character dilutes that metaphor. The Fantastic Four had to rush into space in an experimental rocket so that they could get into outer space before the Soviet Union. By 1990, the space program had basically come and gone - there was no urgency that would have forced a scientist and some hangers-on to launch into space without proper safety measures.
The technology of the period also had bearing on the stories that were being told. How many stories would have been different if characters had access to cellphones, the internet, laptop computers, etc. Concerns about radiation and nuclear bombs factor heavily into those early Silver Age stories, and they are better because of it. If those stories are to remain in continuity they need to be continually updated or rationalized in order to work within the sliding timescale.
Finally, accepting this time scale, there is now an ever increasing gap between the stories that took place in the Golden Age, and the super hero stories of the Silver Age. What was going on in the 60s, 70s, and 80s? Were there super heroes? If not, why not? Nuclear testing and increased levels of radiation in the world originally served as a good explanation for the explosion of super heroes and mutants in modern times. But why would there be an explosion of super heroes starting in the 90s? If there were super heroes, why haven't we ever heard of them? Roger Stern and John Byrne - advocates of the sliding scale interpretation of Marvel time - presented the series Marvel: The Lost Generation in order to address this last concern.
My personal feeling on the matter is discrepancies like this should really just be ignored. Comic book readers are already masters of suspending their disbelief; as long as it isn't actually pointed out, we should be able to accept the fact that Peter Parker was 15 in 1962 but he, and everyone else, has only aged 10 years or so since then. Going in and trying to address it in a comic series potentially causes more problems than it fixes by making a particular rationalization of the "problem" canon.
But nonetheless, it exists, and there's not much i can do about it.
A side note: you'll see that there are some early Dr. Strange appearances mixed in here. That's because he appears in the Lost Generation series; Roger Stern's theory is that Strange had been around some time prior to Fantastic Four #1. There's a few additional continuity inserts that fit into this period as well.