Perhaps i could offer an opinion-
As others have said alot of guys use FR as a step up to better jobs, and ryanair and the likes are more than aware of this, so naturally for the time they work for your company, you want to get your moneys worth.
Whilst not wanting to make a sweeping generalisation, older chaps are more likely to have wives/girlfriends maybe kids and a house with a mortgage. Where as the younguns straight out of flight school generally have no ties and FR can sling them around europe without too much trouble.
All the FR chaps I know have at some point in their career have been a long way from home- and stayed there for a significant amount of time, so perhaps this comes into consideration when Lo-Co's are hiring cadets.