Public Function GetAlter(ByVal sGebDat As String, _
  Optional ByVal sNow As String = "") As Integer
 
  Dim Jahre As Integer
  Dim Akt As Long
  Dim Alt As Long
  Dim Alter As Long
  Dim sTemp As String
 
  Jahre = 0
  If Len(sGebDat) = 10 And _
    Mid$(sGebDat, 3, 1) = "." And Mid$(sGebDat, 6, 1) = "." Then
 
    If sNow = "" Then sNow = Format$(Now, "dd.mm.yyyy")
 
    Akt = Val(Right$(sNow, 4) + Mid$(sNow, 4, 2) + Left$(sNow, 2))
    Alt = Val(Right$(sGebDat, 4) + Mid$(sGebDat, 4, 2) + Left$(sGebDat, 2))
    Alter = Akt - Alt
    If Alter < 0 Then Alter = 0
 
    sTemp = Format$(Alter, "00000")
    While Len(sTemp) < 7: sTemp = " " + sTemp: Wend
    Jahre = Val(Left$(sTemp, Len(sTemp) - 4))
  End If
 
  GetAlter = Jahre
End Function

MsgBox GetAlter("17.07.1983")