Winglets on an older airframe?
Winglets are just as effective on an airframe of any age.
On a 757-200 the fuel saving on sectors above around 1500 n miles is
about 4 to 5%. The price of fuel is the biggest factor in the time it takes
to get your money back. When the winglets were first introduced, fuel
was cheaper then it is now, and the winglet manufactures were claiming
a payback time of around 3 years.
Winglets also are a great help at the extremes of the range of an aircraft,
such as Leeds - NewYork, enabling a few extra tonnes of payload to be
loaded.
So all in all, if an aircraft has around 3 years of life left in it, and doesn't
spend all it's time on very short sectors, it is worth while fitting winglets.