It could be 40 or it could be 50 depending on if NATS thinks it is cost effective.
But the whole point is that you can't make the decision based on cost effectiveness, it's probably no defence against the legislation. It is of course why NATS wants to limit the age and why it's come unstuck in the tribunal. If there are real reasons why older people can't do it then the test for those reasons and reject those that fail but you can't just set an age and assume people over that age will fail, you have to give each individual a chance to try.