As a fellow novice, take my suggestion with a grain of salt. If somebody mistakenly clicks on the Status dropdown for an item that is already "Closed" but they leave it as "Closed" you probably don't want the date to update. You could do a simple check of the OldValue to determine whether or not to execute update of the date. Also, once an item is closed, you might want to put in some barriers to changing the status to something else, but this will depend on your specific needs.
Code:
Private Sub Combo13_AfterUpdate()
If Me.Combo13.OldValue = "Closed" Then
Exit Sub
End If
If Me.Combo13.Value = "Closed" Then
Me.statusdate.Value = Date
End If
End Sub