It isn't clear why, but some adults develop an allergy to a food they used to eat with no problem. Whilst most food allergies still occur in childhood, certain food allergies seem more likely to occur in adults. For example, it’s thought that the average age for developing a shellfish allergy is 50.2 years old. Experts aren't entirely sure why this happens, but theories include: - Hormonal changes may be a factor, including menopause, pregnancy & high periods of stress - Changes in gut bacteria, perhaps built over time from low fibre consumption (starving the bacteria who live off fibre) or a more recent event like surgery, antibiotics, an infection, or again stress. - Not being exposed to a high enough level of the allergen as a child but reaching that threshold in adulthood - Changes to the foods we eat, for example grains are now more refined than ever before and therefore have more gluten. Also foods are sourced from more global sources than even 10 years ago, so the variant may be subtly different. - Natural changes to the body as you age, such as declines in lactase levels, preventing the correct digestion of lactose in Dairy products.