I had a shellfish allergy come on very suddenly when I was 21.
We had taken my dad to an all you can eat seafood restaurant in the Florida keys for Father's Day.. I filled up that day. This was nothing new to me. I'd practically grown up in the Keys. I ate so much crab, lobster and shrimp that my dad would have to pull me away from our friends table.
That night, I went back to my apartment and went to bed. Well, I woke up at about 2 in the morning and every part of my body was swollen. I ended up in the hospital and they pumped me full of Benadryl and lots of other drugs.
Now, the only food I can eat when we go to a seafood restaurant is oysters, clams, steak and fish. Nothing else. I have to notify the waiters about my allergies before I order. It's a pain in the neck, but I would much rather do it than have a reaction.
I carry my epi-pen with me everywhere. I make sure that my friends know how to and where to use it.
So, yeah, allergies can happen at a later age.