[Wrana]

Main argument against the Single Age mod (at least mine and what I've seen here) is NOT a disbalance between different ages' nations. It's that there is a history of Dominions world which describes how various nations changed with time which mostly forbids their coexistance. Lokean offered that they can be something like "factions" or "sects" of the same nation, but this strongly disagrees with what is known for almost all nations (possible exceptions being MA/LA Marignon and MA/LA Pythium). At the same time, there ARE factions of the same nations in the game already. These are Lanka/Kailasa, Pythium/Ermor, Helheim/Vanaheim, plus to some degree MA Man/Eriu.
The only way I see to reconciling Single Age with existing history of Dominions nations is to use a "space-time warp" kind of backstory, where colonists from different times passing through some extradimensional gate(s), end up in the same continuum (by the way, this calls for some really exotic maps, too...)