First as I said before, there was a primal summoned a few year before the fall of the Agrius, in Othard, that event didn't make primals summonable, you could summon them before, at least some of them.

Second, Elder Primals are Primals that have been summoned since multiple Astral Eras without being vainquished. That's why it only counts Bahamut and Odin that have been summoned during the allagan empire Era.

Ramuh was here in the Era of the gods as he says himself and yet he is not an elder primal because he was last summoned only recently. Elder primals are also much stronger because they are here for so long they could suck as much aether as they wanted from the land.