From my observation, most people with food allergies (not mere sensitivities) are millenials.
My theories as to the cause are:
1. The trend toward formula for babies rather than breastmilk. Breastfeeding became less popular in the 80s and 90s as more women went to work, and as a result, more and more babies have had weaker immune systems.
2. Lack of outdoor play. Not only does Vitamin D (from sunshine) aid in digestion and immunity-building, so does activity. I think kids' bodies just are rejecting things that our generation's broke down just by playing outside a lot.
3. Overly processed foods.