It means that when you read your comics twenty years ago or now, some things stay the same and some things have to shift. Originally Reed Richards and Ben Grimm fought in World War II. Then Korea. Then Vietnam. And now Desert Storm.
And you can usually use Franklin as a guide.
However old the child of Reed Richards and Sue Richards is, add a couple of years, and you have however long it's been since Fantastic Four #1 in current continuity. Franklin seems somewhere around 7 or 8, so the Marvel Universe, as we know it, is around ten years old, max.
What does that mean in practice?
The Fantastic Four's space flight, the thawing of Captain America, the setting up of Xavier's school, the arrival of Thor, the kidnap of Tony Stark, the appearance of Galactus and the Silver Surfer, etc etc, all happened in the early 21st century. And it's all post 9-11.
It's probably time for some actual rewriting of the origins of the entire Marvel Universe, a series that threads all those early issues into one post 9/11 narrative.
The space flight as something to give people hope, the mutant fear given the allegory of anti-Muslim feeling and rhetoric, Magneto as a member of Al Qaeda, Captain America reworked in the middle of the Iraq War, Spider-Man never having swung around the twin towers, the fear of the Silver Surfer in that context… oh, and Iran have totally been developing gamma weaponry.
And for those still annoyed that Doctor Doom wept at the collapse of the World Trade Center? It's okay folks, it never happened now.
Also? We are really old.