It's possible to develop an allergy to almost anything at any age -- but what I have noticed in recent years is a sort of synergistic reaction to things. In other words, it's not just one thing -- like cat dander -- but two or three at once, like mildew and bush jasmine and cat dander. (To name three things that are near here, of which two bother me sometimes but if both of them bother me the third will also.)