When you traveled you got into contact with allergens that your body hadn't encountered before. If you had been in contact with those allergens when growing up then perhaps you wouldn't have had an allergic reaction?

>I found that one accepted train of thought now is that people are prone to develop allergies at any age

If you've never had soy before then how could you know whether you have a soy allergy or not?

I'm not trying to convince you either way. Just some holes in the argument.

I'm allergic to grass pollen myself. I have no idea how that squares with the argument, because I certainly never shied away from grass in the summer. But it's a fairly weak allergy.