@Izzy78: I have rheumatoid arthritis and I also work in genetics research. I’m responding to your assertion that "...things that happen in your lifetime probably won't have much affect on it." because it is entirely wrong. Nothing could be further from the truth, and you could be misleading people who come to this article for information.
Any rheumatologist or researcher in clinical and genetic rheumatology will tell you that there are two factors in the disease. One is genetic and one is environmental. Most patients inherit the mutations that can cause the syndrome (it's not a "disease" until a cause is known, and you're right -- we have no exact cause yet, only symptoms and part of the triggers) when they are all (or most are) expressed. But, you can have the genes your whole life and never present the syndrome.
It's worth saying that we don't yet know all of the genes involved in what is called the "pathway" of the syndrome. We know a handful of them, eight of which were only recently discovered in whole-genome studies of arthritis.
Most people who develop or begin to present symptoms of RA and/or other forms of arthritis begin to do so partly because of a prolonged infection or illness, and sometimes due to an injury or prolonged stressors in their lives. This is theorized to push the immune system into a kind of overactivity, possibly causing different immune traits to be expressed, leading to the development of whichever form of arthritis the patient ends with -- in my case, RA -- through inherited traits brought to expression from a long-term infection.
Also, one of the key treatments of many forms of arthritis is suppression of the immune system, either through a combination of anti-inflammatories and steroids (such as prednisone), or more targeted immunosuppressant drugs that have recently been created for that purpose, and in some more intractable cases, low dose chemotherapy drugs are used, such as methotrexate.
The immune system is suppressed specifically because it is the patient's own immune system mistakenly attacking healthy native tissue that causes the inflammation and damage of arthritis. Anything that causes the immune system to become active and start sending out fighting cells, even a simple cold, will cause the arthritis to get worse as well. And of course, with a suppressed immune system, even a cold can be dangerous.
So you see, what you do can most definitely affect your arthritis, even if you have an age-related form and aggravated it by doing too much strenuous exercise. What happens to you in your life and what you do in your life can most definitely affect whether or not you develop it and how quickly it progresses.
If you think you have arthritis, please make an appointment to see your primary care doctor and ask their opinion, and possibly ask for a referral to a rheumatologist if they are unsure or if you would like a second opinion.