Absolute lunacy. If an employer doesn't feel an investment is likely to lead to an adequate return, why on earth should they be made to make that investment?
Common sense but unfortunately probably illegal if you use it to discriminate against somebody. The majority of discrimination legislation would be worthless if you could claim exemption for economic reasons.
It may well be the case that you will struggle to do the job if you start older than 35 but NATS has so far failed to prove that point probably because it can't. The problem with any published age limit is the 34 versus 36 argument put forward by GLA-LAX for which reason almost any tribunal is going to reject a fixed limit as arbitary.
The simplest thing to do is to remove the limit and see what happens. If those over 35 apply we'll get to see how well they fare and gather some evidence to see whether or not the age limit was justified.