I don't see why it should be such a problem, Lufthansa have similar aged planes, as do British Airways. I have every confidence in Jet2's maintenance and just as with every operator, the oldest/most used aircraft will be retired first
Personally I'd much rather fly on one of the older aircraft, but I understand that I'm the minority. Very much looking forward to my NCL-EWR flight on 8th December, and hoping I get 'AB or 'AE, the 2 oldest ETOPS 757's