Nearly 3% of adults in the United States, or 4.5 million people, are allergic to peanuts, according to a study published Tuesday by the Journal of Allergy and Clinical Immunology.; Even though roughly 75% of people in the country with a peanut allergy are over age 17, it is often considered more of a health problem for children, said to Dr. Ruchi Gupta, co-author of the study.