The first 757s joined Monarch in 1983 (I think), probably some of the first in the UK along with the BA examples. You probably won't remember, BBAE, (or were even born!) but one came into Leeds in March of 1984 before the runway extension was finished
and landed and took off with ease.
Yes, they're knocking on a bit but they're built to last so I expect we'll be seeing them for a few more years yet. Mind you, BA have already disposed of their early aircraft and the production line has now stopped so who knows...