ENWorld's been around at least 3 years, maybe 4... if you go back to Eric's original site.
I think 300-400 years is too much. Too many traditions get set in that amount of time, and Mor's End is supposed to be chaotic and new... not traditional.
I was thinking that it was maybe 4 generations from the founding of the city. Assuming generations are separated by 25 years, that would make Mor's End about 100 years old. That's enough time that no humans are around from the beginning of the city, and many of the dwarves that were around are middle aged or old.
Well perhaps it only became this big recently. I imagine if you consider exponential grown, it had about a 100 people after the 2nd century 1,000 the 3rd and finally 10k the 4th. There was a period in the middle when it was almost totally destroyed, but restored by a saviour knight.
And of course no one is quite sure exactly how long the Warrens have been around, or how old What Lurks Beneath is....
IMHO the exact age of Mor's End will be dependent on how the various submissions work out. To put an *exact* age on the city could negate a lot of potentially good ideas.