My dad developed a potentially life-threatening allergy to pennicillin in his 20's, as did his mom. The same has happened to me.
I used to eat peanuts by the fistfull, but tend to avoid them now, not because the peanuts seem to be the problem, but the oils used in processing them. I noticed that when one brand of mixed nuts caused intestinal distress, but another had not (different oils used in processing--Canola and I do not get along).
If reactions get worse with subsequent exposures, I can see where initial very mild reactions might go unnoticed or be blamed on something else (secondhand smoke in the restaurant), at least until the reaction became severe enough in a specific enough situation to pinpoint the allergen.
My mother was allergic to penicillin...but both me and my brother always tolerated it, as children and as young teens...but when I was about 19, the doc gave me shot of penicillin for an infection in my finger...I left the office, paid my bill, went outside, felt ill, went back inside, and the next thing I knew I was on the floor, surrounded by the other patients...a man said, I came back in, look white as a ghost, and then I promptly passed out and he caught me on my way down to crashing on the floor..
From that day on, I never received any penicillin...my younger son, showed an allergy to penicillin when he was about 2 yrs old...he was also allergic to cows milk(thankfully he was breastfed, and the cows milk allergy showed up when I was giving him store bought milk in a cup)...he also had a mild reaction(a rash) to chlorine..
Today he is 31, and no longer allergic to cows milk or chlorine...but he has never had another shot of penicillin..
I have been stung 2 times by a bee...the first time, I did not have trouble breathing, but the site where the bee got me, swelled up much more than it should have...eventually it went down, and I never thought any more about it...however the second time I got stung, I began having a hard time breathing, and again, the site swelled up way more than it should have...a neighbor was about to take me to the hospital, when my breathing returned to normal...
Then I finally realized I must be allergic to bees....so now I have a little medical case, filled with an injector for my thigh, and a few benadryl tablets...
So one can develop these allergies as they age, and by the same token(as with my son), one can also become non-allergic to things they were formerly allergic to...