Elisa,
  I found this in a book "The Milk Sugar Dilemma: Living with Lactose
Intolerance" by Richard and Sherlyn Martens:
"Congenital Lactose Intolerance:  The genetic form is rare since it is
basically self-destructive.  If a child is born unable to digest milk sugar,
he is deprived of any source of nutrition for weeks, which is incompatible
with life.  Usually it is not just lactose which the infant cannot digest,
but also maltose and sucrose.  Today, these infants can be fed alternate,
soybean or synthetic, disaccharide-free formulas.  Many survive in western
nations because of medical sophistication.  We now know that most of these
infants outgrow the problem within weeks or months.  Formerly this could not
be determined, since in premature infants it may become more common as more
of these "premies" survive, mature, and pass-it-along."

Keep in mind that this book is intended for adults with lactose intolerance
and these authors are not pediatricians.
Lezlie Densmore, LLLL in MA