If you ride 2 bikes, are they 'set up' the same for the touch points? - hands, butt, feet?
Problem with both bikes or one?
How fast/slow do you pedal? Cadence.
Flat pedals? Or clipless?
First experience with knee issues?
Pain with numbness? Above patella, below patella?
Does saddle height allow appropriate leg extension?

if both knees go numb, but both return to normal within a matter of seconds (not minutes) that's worth sharing with an MD. Numbness mostly comes from a lack of circulation, which seems unusual in the knees, riding a bike.. Could understand if it was butt or hands or maybe feet.

If it persists, definitely consult a doc.

and join us in 65+ sticky thread in the Fifty+ Forum. You are close enough.