With DC, it's pretty easy to sort superheroics into eras. Just look for the reboot. Marvel's a different story.
Note: I'm not talking about publishing here, but the in-universe Marvel history; so the Timely, and Atlas time-periods might coincide with some of the distinctions, but don't necessarily define them. I'm also not talking about a cosmic calendar, but the history of Earthly super-adventurers.
If we look at the acknowledged MU history, there are obvious event horizons in 1939 and shortly after WWII, and another on the debut of The Fantastic Four. If you were going to sort the MU History of Earth Superheroes into eras, what would they be?