The elderly may be just as capable as the young. Since age is not necessarily an indication of inferior ability or potential, treating a person less favourably on the basis of their age is just as unreasonable and unfair as doing so on the basis of his race or religion. It would also be inconsistent with the principles of equal treatment and non-discrimination, which are at the heart of the notion of individual rights.
If a particular elderly worker truly has, say, less concentration or manual strength than a younger worker, and this objectively and reasonably makes him less qualified for the particular job, then employers can still make their decisions based on his relative lack of suitability for the job – not on his age. Age by itself should not be a determinant.