This all seems to come down to a couple of factors, namely the hygiene hypothesis and the gut microbiome. I suspect that many of the dietary changes which several posters have mentioned as having helped their allergies may have in fact just allowed them to develop a healthier gut microbiome. Overuse of antibiotics, especially amongst the very young, necessarily plays into this, as well.

http://news.sciencemag.org/biology/2014/08/gut-microbe-stops...