AFAIK, Kodak did not age Azo paper. It, like just about all products, was usable off the end of a coating machine. Otherwise, how could we make it. You have to have an aim, not a moving target. This is a common misconception in the photo community, that products age into a 'good' situation. They can be made that way but all to often, they keep going until they spoil.
In my time at Kodak, I know of two common situations where this did happen. Ektachrome Type "R" paper had bumps in the curve due to the use of 9 emulsions. Over 6 months the bumps evened out, but there was no gross change in speed or contrast. It did have a finite lifetime and needed refrigeration. In the second broader case, products hardened with formalin gradually lost speed and contrast and gained in fog. Otherwise they were stable for a few years and gradually went into fog due to heat and radiation as well as the formalin. We see the problems with heat and radiation in some products today.
If a product is built to change, it continues to change. There is no magic signal that goes off in the coating that says "stop today". If it is built to be stable, then it remains stable.
My own Azo type paper is identical the day I coat it and up to 6 months later. There is no significant change that I see. And, I'm not doing anything special except what is expected in emulsion making. I can make it more stable or less stable by treatments common to emulsion making and coating.
Without knowing the M&P emulsion formula, all I can say is that I have seen this type of behavior and found it due to either one of several possible conditions. But I would be guessing if I tried to diagnose it.
PE