I have created a list of 65,000 duplicate Client Identification Numbers
(CINs) out of an original list of 500,000.  The duplicate CINs indicate that
a client has applied for enrollment to my program more than once.  

I have a Date Of Birth (DOB) field, and an Application Date field for every
CIN.  (FYI: most of the clients are under the age of 2 years).

Out of those 65,000 duplicate ID numbers, I'm wanting to determine the age
of the client EACH TIME they apply for enrollment, and I want to be able to
have a SEPARATE FIELD by age category, e.g., "< 1 month", "> or = 1 month but
< 3 months", "> or = 3 months but <5 months", etc.

What I've tried is this:  In building an expression, I've said:  

DateDiff ("d",[DOB],[Application Date]) and in the criteria area (still in
the Design View) indicated <31 (for less than 31 days).  

The response I got from this indicated the 17,000 CINs that had responses
for this category, but I want to keep all 65,000 on the board for the next
field, which would be:

DateDiff ("m",[DOB],[Application Date]) and in the criteria area indicated

What happened is that I only get the response out of the 17,000 CINs, not
all 65,000, and I want it from all 65,000.

How do I build the expression to accomplish this?  I think this may have to
do with Null/Not Null but I'm not sure how to do that.

Thank you!