Yes, you can develop a food allergy as an adult.
Apparently nobody really knows why but a couple of plausible theories:
- being exposed to allergens when the immune system is weakened, such as during an illness or pregnancy
- not being exposed to a high enough level of the allergen as a child but reaching that threshold in adulthood
From the Food Allergy Research & Education (FARE) blog, New Findings in Adult-Onset Food Allergy (the original study is here but behind a paywall):
At least 15 percent of people with food allergies develop the condition after the age of 18, a new study suggests.... The age of first reaction peaked during the early 30s, with patients’ ages ranging from 18-86 years. Another important finding was that an older age at the time of diagnosis was associated with higher risk for severe reactions. In addition:
- A higher percentage of the patients were female, which contrasts with the male dominance of food allergy in children.
- The five most common food allergies among this group were shellfish, tree nuts, fish, soy and peanut. Study participants also identified 14 other foods as allergic triggers.
- Approximately 16% of patients were allergic to more than one food.
A slight aside: While it's really not known how allergies develop (either in children or adults), one possibility is Oral Allergy Syndrome, in which exposure (and allergy) to plant pollen can lead to an allergic reaction to food. That is a little different than anaphylactic shock though.
The chance of this happening to you: This Mass General Hospital article states that "an estimated 4% of the US adult population is food allergic – about 9 million of us". Of that 4%, 15% (1.35 million or 0.6% of US adults) would develop that allergy after the age of 18.
The public health question is really speculative. Obviously it would be very helpful for those people who are either allergic, or at risk of developing an allergy, since they wouldn't be exposed to the food trigger. However, many of these allergens are extremely prevalent in modern food production (peanuts, soy) meaning that food can be produced less expensively. It is probably more practical to emphasize allergens in ingredient lists (as is done in the US) to enable already-allergic or just-concerned adults to avoid ingredients of concern.
