I'm saying that it's quite possible not related to cortisol at all. Or possibly you have the cause and effect backwards. Because of all the wear and tear, we become more cautious about certain activities, which causes more stress, which then causes us to produce cortisol.

Even just the chronic pain of old age (as seen in humans and other animals) would seem sufficient to cause stress.

There's a reason painkillers are such a nefarious addiction; chronic pain happens to everyone eventually and has a profound impact on quality of life.

Cortisol can actually act as a painkiller and reduces inflammation allowing us to work through injury