Can an allergy appear randomly at 20 years of age?

A few examples. 1) your girlfriend or boyfriend has a cat. You didn't have one growing up. 2) you grew up in southern california but are attending college on the east coast & notice the climate & the trees to be very different. 3) you go to a japanese restaurant where they cook at the table. You eat shrimp, a food that you haven't eaten much in the past. Allergies can appear at age 20 but they're rarely random.