The 757 is "old"? What's old?
Years? Cycles? Technologically? In-Service?
In years, they're not that old in "airliner years".
In cycles... well, it depends on how much you use them, doesn't it? Some may be high cycle but industry wide I think there's plenty more cycles in most of them.
Technologically? Probably "old" in the sense that they're pretty much first generation glass and we're probably Gen III or IV glass now.
In Service? HAH! They're YOUNG. They'll be flying along time yet. They're a pretty efficient 180-200 seat airliner and that's what determines how long an airline keeps them in service. Well, that and replacement price.