Thanks for the question. Unfortunately it is nearly impossible for me to make an accurate recommendation on shaft flex without seeing your ball flight/trajectory.
Off the top of my head I would instinctively say A Flex which is also called Senior Flex or Light Flex, but that doesn't hold true for everyone that is a senior. It depends on your strength, speed of your swing, type of clubs, etc..
If you have a golf store near you the best thing to do is go in and ask to be put on a launch monitor or at least find someone in your area that has a swing speed monitor. That would give you a more accurate idea of what flex best fits your swing and most won't charge anything to do it.
Most players do switch to graphite eventually so you might switch to that if you haven't already as they are more shock absorbing and easier on your body and since they are lighter, they will increase your club head speed a bit.
Best of luck and let me know if I can be of help with any other questions.
By: Maria Palozola