Most Jet2 planes are ancient. You pays your money and takes your choice
The 757 is still the backbone aircraft for Jet2 and whilst it is old it is very good. It is still in use because it is powerful and can get out of small airports with more people which means more money. There has been no equivalent replacement until recently.
An older plane cannot really be considered old anyway because the majority of parts will of been replaced several times.
I fly a tinpot Cessna which is about 40 years old and it is fine with no accident history. The only thing on it that is 40 years old though is the structure. All of the controls, the engine and basically all of the moving parts have been replaced many times.
How good would your old car be if you replaced the engine and all the moving parts etc every couple of years?