It's not uncommon at all to develop or even "grow out of" allergies over a lifetime. I know many people who have developed allergies of all kinds at all stages of life. I have also heard that 30-50 is the time when your body tends to make this shift so maybe you're just early. It could also be that you've used up your tolerance to the allergen and are just now having reactions to something your body has dealt with for a long time.