Cow's milk is one the most prevalent allergenic foods around the world, especially for young children. It's estimated that about 2 to 5 percent of children have allergic reactions to cow's milk before they are 1 year old [source: AAFA]. Some doctors tell parents to avoid giving cow's milk to babies until they hit that 1-year-old milestone [source: Sears]. Luckily, most children seem to overcome their allergy in just a few years, after which they can drink milk and have no reaction to it. Occasionally, however, a person won't recover from a milk allergy.
But don't confuse lactose intolerance with a milk allergy. Intolerance for milk results in indigestion, gas and bloating and doesn't affect the immune system like an allergy does.