Citrus fruits -- such as oranges, grapefruits, and tangerines -- are very acidic and can cause stomach upset or diarrhea in babies. That's why doctors recommend that you wait until your child is at least 12 months old to offer any citrus fruits or juices. What's so special about turning 1? That's the age when most babies' digestive system is fairly developed and can handle the acid in these fruits without problems.