I'm in Duke now. Last year, we have a Asian student at age 60. So, it's really a possible thing for your at 40. But you need to have a convincing story in your essay why you want MBA at this time? If you say, you want to try different career, then it will be a big "No". If you say, you have already obtained lots of management experience in your previous w/e, but also understand which area you need to improve for future career development, and you will go back to previous industry or launch your own business, then you're in a much better shape.