atcomarkingtime,
Things might change in the future, but you cannot select prospective ATCOs on the basis that they probably couldn't hack it at LACC or EGLL, but would be ok at Southampton Tower or Farnborough Tower, no disrespect to those units of course.
What happens if a whole course of 52 trainees are recruited that way? EGLF and EGHI get twenty each?
There is a mass of statistical evidence to prove that those over the age of 30 are less flexible when faced with learning new skills, adapting to new environments etc etc than those under 30. Of course, there are always the exceptions before anyone jumps in. That's why the age limit was there in the first place.