My experience has been that this is true. Three things I know that my body now has a different reaction to are:

1. When I was a child in Germany, penicilin was widely used to treat almost everything from earache to brochitis. I had a lot of penicilin as a child with no adverse reaction. 20 years ago (when I was 30) I had an absess on my neck, got treated with penicilin and had a massive allergic reaction to it, consequently I can no longer have penicilin.

2. I was unable to eat tomatoes as a child, I would break out in hives and my mouth would swell. These days I eat tomatoes all the time with no adverse effect, in fact they are one of my favorite foods.

3. For several years during my 20's I developed an allergy to shellfish, specifically langostines and especially in hot weather, I would swell up all over in blisters. I loved langostines, they hated me. Years later my father gave me some antihistimine then asked me try a langostine, hey presto, allergy gone. I eat them in season now and have no problem with them.

Aside from the article mentioned by zebrachick, I was unable to find any other reference to this phenomena on the internet.
Source(s):
Experience

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Yes. When I was a child, I had no allergies at all.

At 20 I got problems (swollen, itchy throat) when eating carrots, apples, cherries, peaches. And I got allergic to cats.

Now, at 40, I can eat peaches without problems, apples most of the time, no cherries at all and I still have problems with cats. I can eat carrots but when I touch my eyes after that, I get a very heavy reaction.
Strange things, allergies.
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This isn't going to be a scientific answer, it's just based on my own experience. I was allergic to strawberries until I was four :( I don't really remember but we found out when I ate one at another child's birthday party when I was three and I had to go to the hospital :s

That allergy just sort of magically disappeared :D

I've had hayfever all my life, for which I ended up in the hospital lots of times when I was younger. I used to get a rash covering my whole body, my eyes, nose and throat would all swell up to the point where if I wasn't in the hospital, I was at home sat on the toilet lid blowing my nose for hours on end :(

That's a LOT less severe now! It's still unbearable during the Summer, but it's a different kind of unbearable and I've found a good antihistamine for it now.

My uncle developed hayfever a few years ago, when he was in his early 30s. It's not too bad now, but that was something he'd never had for the first 30 odd years of his life!

So I guess they can change and they can do it a lot!
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