Hi -

My husband's allergy to casein came on suddenly at the age of 23.  He had
previously developed lactose intolerance (which is rare in children under age 5,
but steadily increases in the population as people get older).  He had no
particular illnesses immediately prior to the onset of milk allergy (unless
you could entering graduate school as an illness :-).  His tolerance has
increased slightly, though.  In the beginning, his tolerance for milk protein
was zero.  He can now get away with tiny amounts (the kind you get through
cross-contamination, or an incautious single bite of a cookie) without
excessive difficulty.

I hope this helps,
--Beth Kevles
  [log in to unmask]
  http://web.mit.edu/kevles/www/nomilk.html -- a page for the milk-allergic
  Disclaimer:  Nothing in this message should be construed as medical
  advice.  Please consult with your own medical practicioner.