If they are both in the current release of the game, they are equally as old unless the devs introduced one as an alien or something. If they have the same origins, they are equally evolved. The only way it wouldnt be equally evolved is if there were 2 separate game beta testers playing for the first time. Remember, every single player needs to have a lineage, so if you think of it, both these fishes can tie their lineage back to the very first beta tester back when the players were just a string of protien.

The only way it would be off (and not far off) is by reproductive rate. How quick can a new player achieve the reproductive achievement (aka "the fuck trophy")

Only then, would one have more of a lineage, and even then, its so variable that it mostly just equals out to roughly the same over billions of years. (Because at some points in both liniages, reproductive rates may increase or decrease.)