Calculate A Persons Age Code Not Working.

Hi all I have another code Problem, Can someone help me with this.
I have two fields
One called cust_birthday ( date field )
And one called cust_age ( text field )
The code below I put in a module name age function.

The Problem, is I get no return from this code.

What is wrong with this code ????

Option Compare Database

Public Function Age(cust_birthday As Date, Optional cust_age As Variant) As Integer
Dim dteBase As Date, intCurrent As Date, intEstAge As Integer
If IsMissing(SpecDate) Then
dteBase = Date
Else
dteBase = SpecDate
End If
cust_age = DateDiff("yyyy", dteDOB, dteBase)
cust_age = DateSerial(Year(dteBase), Month(dteDOB), Day(dteDOB))
Age = cust_birthday + (dteBase < cust_age)
End Function

John527