A person's IQ was tested at age 12 and found to have a full scale IQ
of 40.  She was tested again at age 38 and found to have a full scale
IQ of 75.  What is the accepted or anticipated increase in IQ between
adolesence and adulthood?  I need an opinion from a psychological
professional or acceptable psychiatric source.

Howdy diblaw-ga,

Without knowing if the testing methodology was identical, and as the
testing methodology for testing a person's intelligence (IQ) had no
doubt improved between the 28 years between the tests, as well as
the score differences possibly being attributable to factors such
as basic improved vocabulary skills or improved spatial acuity, I
don't know that one could really make the assumption that there was
an actual increase in the subject's IQ.

Looking Forward, denco-ga - Google Answers Researcher

First of all, you can't get a 40.  A corpse could get at least a 45. 
But that aside, there are several factors that are likely involved
(assuming that the first or second IQ tests are valid):  1.  IQ
increases with education.  2. It is not uncommon for IQ results
measured during childhood to be highly inconsistent (over a standard
deviation or more) with IQ results measured in late teens  or
adulthood.  I cannot remember why, but I believe it is an articfact of
the actual IQ tests themselves (there are different tests used based
on age, ie wechsler scales have one test for toddlers, one for
children and teens up through 16, and one test for 17 yr old and
above).