Yes, back to topic ...
We don't really have a precedent in terms of judging the longevity of airbus aircraft as none of them are as old as the likes of the DC9. We could look at the early A300 and say there aren't many of these around (with pax carriers anyway) but I suspect that is as much to do with poor payload / range capabilities of these early aircraft and suspect economies rather than the physical condition of the aircraft.
Only time will tell whether the A320s last as long as some 737s / dc9s but I'm sure replacing these aircraft is the last thing NW
fleet planners are thinking of at the moment. I'd say they've got 10 years before they even need to think about this and given NWs form maybe a little longer.