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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.