Errr, yes you do...
Err no you don't actually. Gonzo is correct (helps that he actually does the interviews or did until very recently).
...the return on the investment over, say, 20 years is much less than 35...
Correct, but only if you compare two applicants who are succesful and validate - this is not a consideration of the interviewer. They are looking for someone who they believe has a good chance of being successful in training. End of story.
Your argument that if you are older you need to perform better at interview is incorrect - certainly within NATS.
If you are good enough at initial selection and the interview stage, you are good enough.
You are stating that someone has to be better at interview because of the financial implications of return of service. They don't.