I'm 62 and 5'9" with short legs... about 30 inseam. I have no orthopedic issues. I have recently ridden the same exact bike with both 172.5 and 170 cranks ... and I CAN tell the difference. I can ride the 172.5s okay, but prefer the 170s because the longer cranks seem to require a greater bending (more acute angle) of the legs and lower back, especially when riding in the drops. I can also spin the 170s more easily though I am no great fast spinner to begin with.
IMO matching the crank length to one's own body geometry comes first, but the limitations of age and infirmity could come in to play to cause a move to a shorter crank. I can't think of a reason why age would dictate a longer crank but perhaps there is one.