Actually, the RAF does have experience of teaching people to fly over the age of 23. Several times over the past 30 years the RAF has recruited from within. I shared a course in basic training (in 1977) with a bunch of people who had been Suppliers, P Ed, Admin and others - the oldest was well over 30. More recently, the RAF took a number of Tornado F3 navigators to retrain as pilots. On both these occasions, the results suggested that the standard policy was correct - and that they should have paid more attention to recruiting sufficient new blood in the first place!
The Royal Navy has very recently reduced its maximum entry-into-training age from 26 to 23 partly, though not entirely, for similar reasons. On top of that, both the RAF and the RN regularly re-train pilots on new types or variants throughout their flying careers, which can last exceptionally to age 62. All these factors give these organisations a considerable database of relevant and reliable information.
There are exceptions to every rule, and it's quite true that the demands of civilian airline training are not directly comparable to those of the armed forces, however many of the recruiting managers in the airlines you're targetting are ex-forces and so are already disposed towards this point of view. I suspect you'll also find that the HR departments are giving them similar advice. Lastly, it's a salient point that young trainees are more malleable and less likely to complain at bad treatment; I'm sure this is a factor in many airlines' thinking.
Scroggs