Married a Canadian,
A lot has been said on various threads recently about the poor checkout rate that NATS has at various units. That checkout rate is with the age limit being set as it is now.
Correct, but IMHO has nothing whatsoever to do with the age of recruits. It has to do with the cut in training time and content. Before the change to the 'new style' courses, we had an even lower upper age limit (it was 27 when I applied) and the validation rate was fine.
I also still don't understand the difference between a new "young" atco or a new "old" atco. The environment is new to both and they have to adapt accordingly. Why would a 20 year old who is fresh out of school with no experience of anything else be more adaptable than someone who has a bit of life experience.
The statistical difference between a new 'young' ATCO and a new 'old' ATCO is huge. Yes they both have to adapt. That's the point, on average, the 'young' ATCO will adapt far quicker than the 'old' one. The 'young' ATCO is far more receptive to new ideas, is at his/her physical and mental peak....I could go on. Now, yes, in some cases prior experience mitigates this for the 'old' ATCO, but we're talking generally here.
If we are saying that someone over 40 "can't hack it" than there are a lot of unsafe controllers out there.
No, I'm not saying, as you well know! 
In fact, you prove my point. 40 year old ATCOs have, what, 10-20 years of ATC experience and knowledge to call upon to compensate for their diminishing mental capacity, so their overall ability does not decrease. Ask someone to start from scratch in ATC at 40 years old and a very high proportion would struggle.