In the below sentence, we have once used "Their" and then used "his or her". Why?
"Since age is not necessarily an indication of lower ability , treating a person less favorably on the basis of their age alone is just as unreasonable and unfair as doing so on the basis of his or her sex , race or religion"
Their and his and her are essentially interchangeable. You could replace one, the other, or both at either point in the sentence and retain the meaning.
More to the point, you could eliminate both altogether and end up with:
Since it's not necessarily an indication of lower ability, treating a person less favorably based on age alone is just as unreasonable and unfair as doing so on the basis of sex, race or religion.
