I know someone who were able to eat shellfish throughout childhood but then developed an allergy to it when they got older. why does this happen?

Antigenic stimulatn: Food allergies can all be acquired during your lifetime if your system responds that way.Any food product is composed of a variety of proteins, carbs or fats which are broken down into subunits during digestion and absorbed.The protein subunits may trigger a buildup of antibodies just like you do to vaccines. In time the antibodies may reach a quantity that causes allergy symptoms.