I don't think it is as automatic of a connection as you are saying, and haven't seen any article that claims it makes any difference for people in their 40s. (The articles are all talking about elderly people.) The research is just incomplete. Some of the studies that have drawn this conclusion have sample bias, and while some are willing to suggest a higher risk for BD patients in later life, there are a lot of caveats. (Also, of course, remember that you have a chance to have dementia later in life whether or not you have BD.) Before reading and freaking over things on the Internet, ask your doctor for better insight.