Try;
Public Function AgeGroup(dtmBirthDate As Date) As String
Dim intAge As Integer
'Age Calculation
intAge = Int(DateDiff("d", [dtmBirthDate], DateSerial(DatePart("yyyy,Date()), 1, 1))/365.25)
Select Case intAge
'For each Age range, write out Age Group (used in qry)
...
End Function