I, too, find age renders some foods less tolerable. I thought I knew which foods were bothering me, and I kept doing experiments to try and prove it (eliminating X or Y for a week). But I didn't get a true picture until I did the FODMAP diet. I can't recommend it formally, as a journalist, because the app kinda tricks you into heading down a path that leads to a money-making thing, but if you avoid going down that path (and it becomes obvious, so you'll see) the basics of the program are enlightening. It's a simple reductionist plan where you strip your diet way down then add things one at a time (it does requires some serious commitment and record-keeping for a few weeks). I found foods that definitely cause GI issues but which I had no idea would. And I found that some I thought bothered me do not. And I discovered some tasty and helpful alternatives to replace things I should not eat. You gan find it via a routine search.