You'll probably see color fidelity start to lapse as the developer becomes exhausted. As for age, I generally find that C-41 developer lasts roughly six weeks in plastic Datatainer bottles, but others have posted longer lives in different storage.

When I mix up developer and have a stockpile of films to process, I save any chromogenic B&W (Kodak BW400CN, Ilford XP2 Super) films toward the end, since color accuracy is of course not a concern.