What species of quail are you talking about? Bobs can generally lay for longer than a coturnix. I agree with GrandmaBird, if you want to tell them apart leg bands are the best way. If you don't have access to them put your older birds in one cage. For most of my quail I keep only one age bird in a cage so I know what age all of them are by just writing down when the birds in that particular cage hatched. The cages with mixed age birds are different colors so I can tell that the brown quail are a different age than the white quail in the same cage. I've got to keep is simple.