Yes it is true that as some children get older, their food allergies will simply, go away. Some foods in infants can cause rash, wheezing, diarrhea or vomiting. Most of these children will outgrow these reactions as their immune system grows stronger. Occasionally, these food allergies can follow you through adulthood.
Some of the most common allergies to proceed into adulthood are: milk, eggs, wheat, soy, shellfish, peanuts, and tree-nuts. Milk, eggs, and wheat are often outgrown, but peanuts are for life and can be very serious.
If your child is having weird reactions to things, it may be worth a trip to an allergist. Even things like oranges, and bananas can cause mild anaphylaxis. The best thing to do is normally avoidance. Food allergies are normally accompanied by other allergies. These can be a real hassle dealing with but there are medications that can help.