The whole point I am making is that it should not have to be "good" or "justifiable". In fact, there should be no need for any reason at all
Why not? Age didn't gain protected status for no reason. Older people have the same right to work as younger people.
It is none of our business. Both literally and otherwise...
I understand what you are saying. You want business owners to have the right to build whatever team they want, and there are any number of reasons for that, not all of which are discriminatory. Maybe you just want a younger looking public-facing team because you think it makes your company look fresh and cutting edge, and that's valid, but too many employers have demonstrated that those decisions sometimes ARE used to discriminate against older workers, and so the government stepped in. If you don't understand why the government stepped in, then you haven't been passed over for someone younger (yet).
We're not even talking about how this affects the workers. Imagine if there were no age discrimination laws, and all the companies in your area started grabbing all the fresh young college grads, and stopped hiring anyone over, say, 38. Now you have a whole population of perfectly qualified, willing workers who can no longer find work, simply because they were born earlier. How fair is that?