Adam - the answer is, no-one knows! That said, it gets harder to pass the FATs and get through flying training if you are starting aged 25, as aptitude starts to fall off from aged 23 onwards, roughly.
That said, the guy from the course after me who got the only Harrier slot from his EFT was 25 and a qualified barrister when he joined the Dark Blue. Charlie Gilbert - now a large cheese in BAe.