Just looking at the old JP and Curious Pax is quite correct; Jet2's 20+ are almost all (with one exception - and that is a cargo acft) from '86/88. BA's 737-400s are all early '90s, only '88 vintage aft being a few -300s and they won't be around for too much longer.
KLM's 737-300s are late '80s aircraft, but I'm presuming that these are going to be replaced by the -700s, already entering the fleet.
Lufty's 737s are all around the '88-91 timeframe too, so it does indeed seem that Jet2 has the oldest average of any European 737 fleet.