I've never experienced this before and perhaps I'm just a "victim" of getting a bit older. I am in pretty good shape 5'10" 168 lbs 33 years old.
I doubt it is age.
I am 64 yo, and rode 169 miles last week - and have absolutely
no knee problems.
Perhaps seat height? You might experiment a bit.
Go see a sports ortho specialist if it keeps up.