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.
Thanks for the explanation guys. This is something I've always wondered about as well.