I can't speak for what happens at Manchester, but if easyJet are seriously claiming that their average fleet age means they are more reliable, the dreadful performance at Gatwick strongly suggests otherwise.
BA's 737s are an average of 18 years old
easyJet's fleet average age is 3.7 years old (so they say)
You were three and a half times more likely to be delayed flying with easyJet from Gatwick than with BA over the last two months. And if you were delayed, the delay with easyJet would be on average three times longer than BA.
easyJet ought to be hanging its corporate head with shame at the current performance. A modern fleet does not lead to good punctuality if incompetently managed.