Peanut allergies began to decline in the U.S. after guidance first issued in 2015 upended medical practice by recommending introducing the allergen to infants starting as early as 4 months.

Health experts quickly took notice — and the resulting reversal in pediatric guidance has helped to push peanuts out of the No. 1 spot as the cause of food allergy for children under 3 in the U.S., ac...