VERY rare. True lactose intolerance is very rare in children, especially since lactose is the primary sugar in breast milk. In 15 years I have seen 2, count 'em, 2 children who were lactose intolerant. Much more likely is cow's milk intolerance or even a mild milk protein allergy. Kids will outgrow both by around 9 to 12 mo of age. A soy formula or a "sensitive" formula help avoid the protein intolerance.