DTY/LKS - Commonsense posting.
A similar situation applies with medical standards. The requirements for passing an initial Class One are far more stringent than those for an experienced pilot/ATCO of advancing years. A pilot/ATCO of, say 50, could pass his renewal medical with no problem yet would not necessarily pass an Initial.
I trained controllers at home and abroad for most of my career and I firmly believe that NATS is right. If they are prepared to train at great expense in the expectation of employing somebody they must be confident that they can post that person to any unit. Sure, I've seen middle-aged ATCOs validate at busy units but they had much experience to fall back on. On the other hand, I've seen a good few middle-aged ATCOs moved away from busy units simply because they were having age-related problems.