If you knee motion or the efficient portion of it is compromised with age, then shorter cranks make sense. I would change to proportionately lower gears to compensate for the lost leverage. Having said that, I really don't notice any difference between the 165mm cranks on my Peugeot and the 170s on my other bikes.
__________________
"Far and away the best prize that life offers is the chance to work hard at work worth doing." --Theodore Roosevelt
Capo: 1959 Modell Campagnolo, S/N 40324; 1960 Sieger (2), S/N 42624, 42597
Carlton: 1962 Franco Suisse, S/N K7911
Peugeot: 1970 UO-8, S/N 0010468
Bianchi: 1982 Campione d'Italia, S/N 1.M9914
Schwinn: 1988 Project KOM-10, S/N F804069