The precise causes and mechanisms leading to OA are still not known. Like many unsolved diseases, it is probably due to a combination of many factors rather than one single cause. Age alone would not cause it unless you lived to be 200, but ageing may make a contribution. However a major injury or an operation on a joint may increase the risk later in life. This is particularly true of the sort that affects the hands of middle-aged women.
Heredity plays only a small part in most other varieties.
There must be other causes, but we don't know what they are yet. It doesn't help to be overweight, but OA is not caused by diet, exercise, the weather or a shock. It is very common to see it on X-rays. About half a million people in this country are affected but many never know or suffer any pain. It usually starts when you are in your fifties, but can begin at any time from the age of thirty.