I notice a decrease in my flexibility as I age. I have managed to keep or even increase my strength in most ways but can not twist or rotate as well as I could ten years ago. That does not change gun fit but does require alterations of how I shoot. Better foot work and careful gun mounting is all I have to do for now.
Unless you are having major changes in your shoulders or neck region I suspect any fit differences are minimum. If you have lost weight, a thing many wish for but can't do, fit should not change that much. You might just need to alter how you shoot slightly. Or get fit again and buy a bunch of new guns. Always a good excuse to buy a gun or custom stock.
