No, 24 is the start IOT limit (on your birthday) 23 1/2 is a practical limit for applying at AFCO but is not to say that 5 months to go or even 4 or less would be too late.
Also no longer are you only considered for the branch to which you apply. Similarly you will undergo the full assessment process. If, at the end you are not offered a place in your chosen branch then you may be asked to consider other choices.
In your case Victor we might deduce that the RAF has a need for Fighter Controllers and Air Traffic Controllers. Like aircrew these are aptitude related jobs with similar skill sets to aircrew. Naturally if they fill up the aircrew slots and the 'extra' aircrew candidates depart it follows that ATC and FC will run short.