Different Age Limits
It's a combination of aptitude and population size. Say the peak aptitude for Aircrew occurs at about the age of 22/23. There will be some people who have the aptitude at 18 and some still who have it aged 28, but in both cases, many fewer than at 22/23. The RAF has very many more applicants that the RN, even though it also has more places available. It is also more likely to be the first choice for many candidates. So the RAF can fill their slots by only looking at the age range between 18 and 23. However, as the RAF snaffles most of the candidates in this age group, the RN has to cast it's net a little wider and look at candidates up to the age of 26. The RAF could put its age limit up, but that would be inefficient, because, for example, the pass rate at 26 might be one in 200 compared to one in 20 at the age of 22.