In a recent interview for a graduate school program the interviewing
faculty member asked the age of my wife. I don't see how this could be
relevant to my enrollment in the program. Is this relevant to a
discrimination case?

Yes, the question was inappropriate, but what was the context?  Was
the interviewer making conversation?  I don't think this constitutes
discrimination.  WOur society needs to stop being so litigious (too
many damn lawyers, I guess).

I don't know what the law is for college admissions, but this question
would definitely be illegal for a job interview.  I am only guessing
here, but perhaps the interviewer was trying to find out if you a baby
was in your near future since this could be distracting from your
studies. If you end up not getting into the program and you don't care
about burning bridges, you may want to send a letter/email to the
interviewer with copy to a dean of the program specifically asking
about this question and its relevance.