That's a hard question to answer because no one really knows. There are, however, differences in the approach to child rearing that may have an impact. In my young days, that were post WWII, children were given everything to eat and there was no thought of allergies. The closest my body came to one was that of dust from which sinus would result. No one in my family or anyone we knew, however, ever mentioned allergies.
Babies were returned with a taste of everything, even when they spat it out. Eggs were high on the menu as were nuts, fruits, vegetables of all types, and, of course, meat. My children were given exactly the same treatment. My babies received cows milk when they were weaned and nothing was held back by way of food.
My daughter, by way of contrast, read books to determine what her children should eat. They were denied eggs and lots of other foods. The result, which may have come from this practice, is that one of them is highly allergic to certain foods.
The other factors that entered the scene are books written by male doctors who stressed the importance of holding back on some foods for a young child. This coupled with the junk food that replays the fresh or home cooked meals sees young children given things like soda drinks high in sugar, foods high in salt and other things, and sweets, such as iced coated donuts and biscuits.
Watching young mum's feed these things to toddlers in shopping centers to keep them quiet while she concentrates on her purchases leads to other thoughts. Why take a child that young into these places where they see the junk food and scream their heads off to get them?
It may be hard for mothers to work their way through modern trials and errors to avoid health problems, but they need to wake up to what modern takes on child-rearing are doing. While such things may not be liable for allergies it might be interesting for some practitioners to examine the present changes in this area compared to post war years. It may also help to understand the obesity problems that faces some adults.
Surely what children grow used to follows them into adult-hood. That may apply to those who can not walk through a shopping center without first visiting the junk food outlets. Interesting thoughts that should be examined further.