The Far Realm's connections to the multiverse of the Great Wheel are few and tenuous, and the physical laws and concepts of one may have little or no meaning in the other. Time as understood in the Great Wheel may be irrelevant in the Far Realm, and as Rip said, the Far Realm exists outside of our concept of time.
But, that said, it would be wrong to consider the Far Realm something that the Wheel floats within, or that the Far Realm is outside and surrounding the Wheel. There's absolutely no physical overlap between the two. For all anyone knows, the Great Wheel and the Far Realm are both seperate, incredibly different and divergent multiverses each with their own rules and predominant concepts. Since time began in the Great Wheel within the Great Wheel, there's no objective 'time' to measure age of the Wheel versus that of the Far Realm, or any other seperate multiverses that might exist.
So "which came first" or "which is older" isn't really a relevant question that can be answered.
Presumably whatever interaction between the two might happen at random points in time, such as Piscaethces briefly interacting with the Wheel and spawning the Aboleths perhaps by accident shortly after the formation of the Wheel's prime material plane. Another such interaction on her part need not happen after that first one within the rubric of time in the Wheel, because our concept of Time holds no meaning for her/it. Our idea of time is just as alien and horrific to them as theirs might be to use.