I don't think there's much an issue with the fleet age. Sure they're going to be reaching EOL with some of the 733's and I suspect also th 752'S in a few years but I don't think they had more issues than any other airline. Look to Monarch (awaiting ciampino to attack me for this) - with good solid maintenance, aircraft can last to their limits. They'd do better holding out for another 2 years before renewals - I suspect plenty of young 738's to be on the market by then which would be a good fit for jet2.
Edit: they could probably also put their old 733's in for cargo conversion in a few years too - will allow them to boost cargo ops.