I'm almost 68. I ride a C & V bike with 45/42 chain rings and a 13-15-17-20-23-26 freewheel in San Diego's northern coastal suburbs. I often use the 42/20, but shift somewhat frequently.
With a 144mm BCD crank spider and a short-cage SunTour Cyclone II rear derailleur, 42/26 is about as low as I can go, and it currently works for me.
The day I need to go lower, I plan to keep the half-step and the chainrings, but change to a 7-speed 13-15-17-20-23-26-30 or possibly ...-32.
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"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