If you consider the punishment your knees get, and that's just by supporting your body weight day in day out, it's hardly surprising that knees, along with hips are first to suffer in later life. I know you like to do a bit of running, and you mentioned in the "fitness and health" thread that you were getting a bike too. That's a good move imo.As someone who enjoys having knees capable of jumping down a mountain, a long life of immobility would suck.
I'm 52 and a keen biker. My knees are fine at the moment. In fact the only time I have problems with my legs is when I do a bit of running during the winter months when it's too cold to ride my bike, and too boring using my indoor trainer. You are blessed with better weather than us in the uk, riding a bike year round as exercise will be more beneficial, long term, than pounding your joints running.
That's only my opinion though, I know there are runners on here who'll disagree and that's fine.