Aging-related inflammation is not universal across human populations

We do not really know what causes misdirection of the immune system in the old age. "Boredom" (= underutilization) is just one of many hypotheses. Differences between lives of jungle dwellers and urban people are just too big to pinpoint one root cause (such as parasitic infestations). Various chemicals may also play a role, as may artificial light, differences in food composition etc.

Let's say you're eating some wheat. You have no issues with it.

One day, you eat wheat that was contaminated with something, call it thing X. It causes a reaction. Yet the reaction now targets wheat and thing X, and so wheat is now troublesome.

Unsure on the validity of this.

Gluten is like glue. It's possible that your digestive system is simply no longer able process glue, cause it's kinda hard to do.