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.
The responsibilities of an employer are prescribed by law, and may not include all those things that you would wish for. However, this is not an argument about an employer's responsibility to his employees, but about what methods an employer can use to ensure he gets the best employees for his purposes.
Where there are no other factors, age should not be used as a reason for not employing someone - although there is, I believe, no law that says so. However, as the RAF has discovered after 80+ years of training pilots, age is most definitely a factor in the success probabilities of student pilots. As they don't have unlimited time and money to throw at slower learners, they restrict entry (for pilots) to age 24.
Nevertheless, you will discover, if you research further, that most airlines' maximum age for employing pilots is around 5-7 years younger than their notional retirement age, i.e. about 48-55. That is to allow the individual a reasonable chance to advance to command, and for the airline to amortise the training costs (for an already-experienced airline pilot). It is only the entry into sponsored ab-initio training that is limited to young people. It's not discrimination, it is risk management.