From a previous thread somewhere here, I remember it to be something like capital cost vs operating costs. Older jets have higher operating costs but much lower capital costs. Thus the economics for a plane that is only flown a few hours a day on average (no I cant define few!!) favours higher operating costs and lower capital costs. The more the plane is flown per day the more important operating costs become and thus favouring newer planes.
A good example of this is Atlas, where a fleet of Classics provides charter services while a fleet of -400s operate scheduled service. It is all about utilization, and the balance of fuel burn, maintenance and capital costs.