Do our joints become weaker as we grow older? What can we do to prevent against joint problems in old age?

Thanks you for your question and sorry to hear about your knee pain. There are many different causes for knee pain but a common problem in your age is something called patella-femoral pain syndrome. This is usually from a relative muscle imbalance and hence the phenomenon of mal-tracking and pain. In general this can be resolved with rehabilitation - but it would be important for you to seek a formal assessment with investigations if appropriate. I would suggest visiting a Sports/MSK physician or physiotherapist in the first instance. They can evaluate your knee and advise accordingly.

You do sound like you are experiencing a mix of unusual symptoms and perhaps it might be worth seeing a neurologist with a special interest in myopathies and neuro-muscular disease. Hope this helps.