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.
Good points, and I agree.
One additional point .......... for most, aging occurs slowly. As we gradually lose strength, flexibility, etc. we adapt, IF we shoot regularly. There may come a time when I can't shoot my big 9 lb.+ MX8 and HE Fox. But, for now, at age 66, the only difference I note is that when there is trap trouble and the birds are coming out broken, etc., I can't keep holding the P gun up ......... I have to dismount, rest a few seconds, then move ahead.
I wouldn't make big changes in gun fit until my shooting really changed for the worse. I think you'll be able to identify what that change needs to be, if you shoot enough. Likely it will just be a lighter gun.