I had no allergies until I was away from Minnesota in the Navy for 3 years. I lived in Florida. The first year that I moved back to Minnesota I became allergic to the pollen from hardwoods in the spring. I was 23 then.
Then at age 46 I developed the 24/7 year round allergy - The lightheaded, headache, nausea, cough, clearing of throat, tightness of chest deal. That was 4 years ago and the Chlorphen (generic for C-trimton) was the lifesaver for me.
According to my allergist, the older you get the more likely that you will develop additional allergies.