My father in his mid-to-late 20s didn't need anything for allergies, food or otherwise. He would work outside undeterred in the spring. Any nice days the windows were open for "fresh air" during the spring, even as the rest of us begged for them to be closed (pollen/sensitivity).

In his early 30s, something clicked, and it was all different. He became very allergic to a variety of seasonal allergies. Wear a 3M disposable respirator when blowing the leaves or mowing the lawn and goggles, different pills/eyedrops/nose sprays (all under doctor's orders, not stacking conflicting meds), absolutely miserable. Will get testy if he suspects a window *was* opened during allergy season.

He's gotten so bad now with allergies to certain trees that he no longer can eat certain fruits. About ten years ago he ate a peach in his office (small, in the corner of a warehouse, alone 80%+ of the time) and he popped the pit in his mouth. His throat swelled almost completely shut. He drank ice cold water along with taking Allegra and thank god he survived (did not have epinephrine, and still does generally not carry it).

My older brother developed stronger allergies to trees in his late teens (18-19) and can no longer eat fresh apples. He can eat cooked apples in pastries or pies, since that apparently denatures the proteins enough for oral allergy syndrome to not be a problem.

I developed a pecan allergy in my early 20s (although I test negative for everything now) and every single person in my family has now had a reaction to hazelnut except for me (all in adulthood... I don't eat hazelnut anymore.)

For seasonal allergies, what I've observed personally, I'm convinced. For others... not so sure, but why should environmental and non-environmental allergies be any different?