A320ECAM
The oldest of the B737-300 are just under 31 years old, however some of the B757s are only a year younger.
You need to realise that some of the worlds B737-300 are in fact younger than some of the B737-800 Jet2 operate.
Providing the aircraft are maintained correctly as Jet2 do so, airframe age is irrelevant, the issue is more down to components that fail more with age, which is most likely what happened on the incident recently.