[Endoperez]

Well...

EA nations often have big, expensive, specialized commanders. They start with income of 6 gems.
MA nations sometimes can summon big, expensive, specialized commanders, especially if the version from earlier era had them. They start with income of 5 gems.
LA nations are rarely diverse in magic. They start with income of 4 gems. Death seems to be more common in LA.

Some nations would be pretty easy to move from one Age to another. As an example, I think Patala could be made into an EA nation simply by replacing their armored apes with those Kailasa has. There are few reasons for not doing that, but their mages are expensive and powerful, and I immediately thought of EA when I noticed them.