According to the Cleveland Clinic, anyone can suddenly develop an allergy to shellfish. They report that a shellfish allergy actually is more common in adults and older children.
The Cleveland Clinic states that an allergy to shellfish can be caused by previously consumed foods, even if a person never had an allergic reaction to those foods. Although it is not always the case, people who are allergic to one type of shellfish are often allergic to other types of shellfish. Symptoms of a shellfish allergy include itching, hives, eczema, swelling of the face, difficulty breathing, stomach pain, dizziness or fainting. In some cases, anaphylaxis can occur, which is dangerous and potentially life threatening.