Hi,

I'm basically all for spinning and cautious about mashing though I've never had
any trouble with my knees, or anything else, I push all the time, but not too hard.

Optimum cadence for efficiency does fall slowly with age.
Optimum cadence for performance is an entirely different kettle of fish.

Mashing builds strength and power, spinning builds endurance and power.
Most are interested in power output and both ways help develop it.

If you can do 75rpm in a gear, dropping to 60rpm is not mashing, its
going slower with less power, and less tiring. Going up at gear or
two for the 60 rpm you are then mashing compared to "normal"
cadence, and I doubt it does any harm indulged in moderately.

I've no idea if at my age I'd develop knee issues riding around
on a SS (fixie) and no intention of riding my bikes in a style
that would find out.

FWIW as group rides tend to be faster due to aero advantages,
the same advantages make it easier to push higher average
cadences than you would riding solo.

I don't have a cadence meter, but have speedometers.

Top gear on the folder is 15mph @ 80 rpm and 19 mph @ 100rpm.
On the flat no wind 15 mph is mild pushing, 19 mph pushing quite
hard, flat out I've hit 21/22 mph but not for any real distance.

I'm pretty sure I ride the road bike most the time 80 to 100 rpm.
Slower on some hills. Tanking up up short sharp hills on routine
rides in high gears I'm sure is helping compared to just going
up them spinning away, I get plenty of spinning practise.

rgds, sreten.