As the one paying, the employer is the customer. Like you when you wish to purchase something, they are entitled to set their own criteria when doing so.
Although I cannot fault your logic, the problem is that purchasing goods means purchasing inanimate objects. Employing people is not only a moral, ethical and implicit contract of responsibility to the employed person (and visa-versa) but also to that of society! Discrimination should be limited to a person's functional ability to deal with the job in question. Age most definately does not fall into that category. To say that older people do not respond to training because of their age is complete bullsh*t
People do not respond well to training for only two reasons: 1) Their ability 2) Their motivation!