Well, it is surprising to me! I know people tend to become lactose intolerant as they age, but not to other common foods.
In my family, drug intolerance develops toward penicillin some time during/after puberty.
That isn't so surprising, when you think of the biochemical changes the human body undergoes at puberty, and circa menopause, not to mention pregnancy. With a changing biochemical situation, I would expect that some things would become less friendly, perhaps reflected in changing taste in food in general, with changing dietary tolerances and needs.
I have seen grand kids who would not eat eggs or touch peanut butter become avid fans of both, after they hit their late teens. My mother developed her allergy when we kids were fully grown...so that fits.
If I had to guess a single factor, though, she had a reaction to Starlink corn, which had made its way into some taco shells she had eaten. She had had a coupole of different reactions, and the doctor asked her what pesticides she had been using (she hadn't). The doctor couldn't figure out what was going on, and she had been to the ER twice. When I talked with her I had read about the Starlink corn getting into a batch of consumer products and checked. They were the right brand, she threw them away, end of reactions.
https://en.wikipedia.org/wiki/StarLink_corn_recallSince then, though, there has been a shift in the crops grown in the area, from tobacco (only the Amish grow Tobacco in Southern Maryland, now, a legacy of
The Tobacco Settlement) to grains, mainly corn and soybeans. The seafood in question has come from estuaries adjacent to the farm fields where tobacco is no longer grown, and there may be some transfer of proteins from the corn corps to the seafood--a link I can only postulate, and which would require some research to verify or refute.