It varies depending an a bunch of factors-
Example, I'm nearly 60, 50 lbs. overweight, bad back and arthritic knees.
I ride a 21 year old MB with street tires and 175mm cranks.
No way can I spin a cadence of 70 without bouncing all over. I can't/won't use any kind of clips because I have to keep shifting my feet slightly to avoid knee pain.
I recently 'discovered" I was doing a cadence of 60. A bit of experimentation and I found out I could downshift a gear and do the same speed at a cadence of 65. MY stamina is much better at 65 and "spinning", than at 60 and "mashing". I'm looking for shorter cranks. That may allow me to spin a bit faster. (and then, probably a different cassette with closer spacing)
Somebody else may be best off at 90 or whatever suits them.