Just some ideas.
Dim noDOB As Integer
Dim tAge As Integer
Dim tYears As Integer
'show a zero for null birth date value
noDOB = 0
'age of person in years old
tAge = (Year(Now) - Year(Me![txtBirthDate]))
'excludes current year from years old; however, this will result in an inaccurate years of age.
tYears = (tAge - 1)
If IsNull(Me![txtBirthDate]) Then
Me![txtAge] = noDOB
Else
'Not sure why this next line needs to be here. Simply append the tYears to [txtAge] if [txtBirthDate] is not null
If DateSerial(Year([txtBirthDate]), Month([txtBirthDate]), Day([txtBirthDate])) < Now Then
Me![txtAge] = tYears
End If
End If