I know an individual who developed a milk allergy in her early 50s. She grew up drinking it and eating cheese and all kinds of milk products. One day a switch flipped. Milk fat (butter, etc...) doesn’t seem to bother her. Just the protein. She’s trying to figure out if it’s the whey or casein. It’s looking like the casein is the culprit.
My allergist said that anyone can develop an allergy to anything at any time in their life for any reason. It’s seemingly completely random.
That said, gradual desensitization can work wonders.
I know someone who was deathly allergic to peanuts and yet by sublingual drop therapy can now tolerate small amounts of them so that accidental exposure wouldn’t risk killing them any more.
I would not have thought it was possible to give someone that severely allergic a small enough does to trigger and appropriate immune response and retrain the immune system to not react inappropriately.
But this is someone I know personally, so, yes, I believe it now.