You can look at retirements at each base...but this doesn't factor in pilots changing bases.

Here's my prediction.

PHX - senior because when it opened, many pilots flocked there, who are now retiring. Phoenix doesn't have the same draw it once did, so as these pilots retire, it will return to a more average seniority level.

ATL - senior because of the AirTran merger. Since all AirTran folks can hold ATL as an FO, it's no longer really senior for FOs like it was 5-6 years ago. Overtime as AirTran pilots retire, it will also return to a more average senroity level, but over a longer timeframe than Phoenix.

MCO - will always be senior. What happens at MCO it that MDW/BWI Captains get near retirement, get tired of the cold, are empty nesters, and get tired of taxes. They decide to spend their final years in Florida. There are Captains at the bottom of the MCO list that haven't moved relatively in a long time, as more senior pilots bid in above them.

DEN - Not to senior now, and due to it very likely becoming the largest domicile, will become some what more junior. Many pilots flocked to Denver when it opened, but it's not as attractive as it once was.