I work at Jet2 and can HONESTLY say that the aircraft are flown and maintained by well trained professionals who take a great deal of pride in their work.
The aircraft may be older generation but unlike perhaps your average car get serviced and checked constantly. This means that while the basic structure of the aircraft is original all the "working bits" are replaced on a rolling schedule so while one part may be three years old another may be brand new that week. Its the same with all aircraft that are correctly maintained.
If it helps to put your mind at rest, my family ( wife, kids, grandparents, in laws ) regulary use Jet2 because I know that they will be in good hands whilst at 35000ft.