All I know for sure is that all the listed ones (Ul'dah, Sil'dih, Gelmorra, Gridania, and Ishgarud) all originate in the 6th Astral Era at various points. Even Ishgaurd being in a 1000 year long war would still only put the war starting between 600 and 700, 6th AE.

But I'm willing to bet that Limsa is likely the youngest as it started as more of a "Pirate Hideout" before it was made an actual city. The Rouge quest says that there was alot of unrest in it's early days until "The Code" was written. And that "The Code" Governed Limsa long before the Thalassocracy did. So it seems to suggest that Limsa as an actual city state is fairly recent compared to say Gelmora and Belah'dia who's founding can betraced back just after the end of the 6th Umbral Era.