Access Cancel update of Date field if not valid on bound form

Hi. I want to use the following code to check whether a date entered is valid.How do I cancel the update of the field if the date entered is not valid?

Private Sub DateofBirth_AfterUpdate()
    'Test if applicant is under required age.....
    Dim CurrentAge As Integer
   
    If oCountry = "Samoa" Then
        oModalAge = 18
    ElseIf oCountry = "Tonga" Then
        oModalAge = 21
    ElseIf oCountry = "Liberia" Then
        oModalAge = 18
    ElseIf oCountry = "Fiji" Then
        oModalAge = 21
    Else
        oModalAge = 18
    End If
   
    ''CurrentAge = AgeYears(#12/31/1985#)
    CurrentAge = AgeYears(Me.DateOfBirth)
    If CurrentAge < oModalAge Then
        MsgBox "Applicant must be at least " & CStr(oModalAge) & " years old!", , "CAVR - ATD": Exit Function
    End If
End Sub