My suggestion, if you're able to do it, is to regularly practice the full extension of your knee joints. By this I mean, from a leg that is absolutely straight to one that has the knee joined bent to its maximum. Further to this, I suggest that you hold each of these postions for sixty seconds or longer. Do this a couple of times a day for a while, thence once every two or three days should suffice.
The idea here is to keep the knees ligaments strong and flexible and to keep the blood circulating through the knee joint so that any build up of calcium deposits are reguarly washed away with the blood flow.
In effect, you would be holding Hatha Yoga poses. You might not be able to get the full range at first, but you could keep at it regularly until you do. To explain a bit further: Sit on the floor, legs outstretched and reach and hold your toes for sixty seconds. If you can't reach your toes, try the knees for a few days, then the lower leg area, gradually stretching further each time until you can hold your toes. That is pretty well a full leg extenstion.
To get its opposite, sit on your heals on a comfortable mat. Sit and hold this position. Gradually get the the stage where you can sit 'between' your heels. This a full bend of the knees. So with these two expercises, done regularly, I'd expect the stiffness you mention will gradually go away.
Don't over do it, of course.
I am not a medico. However, I am seveny-eight in April and have had no knee trouble. I expect it is because I pratice these stretches myself and have done since I was thirty or so. Regular stretching will keep you supple and with less likelihood to get those aches and pains.
Good luck with it.