This answer stems from a few comments I made to a related question.
Are old (>35) faculty candidates discriminated against all over the world?
I can't speak to the worldwide aspect of the question. What follows will be specifically tailored based on my tenure-track job hunting experience in the U.S., although it may be applicable in other parts of the world.
As pointed out in another answer, it is illegal to discriminate on the basis of age in the United States.
However, I think the more interesting question to ask/answer is:
Even though it is illegal to discriminate against someone due to their age, does that mean that age plays no role whatsoever in tenure-track faculty hiring decisions?
Sorry to get your hopes up: I don't have so much a concrete answer to this question; however, I offer an example of something an "older" candidate may face when seeking a tenure-track faculty position.
Being an "older" candidate myself, I noticed on my unsuccessful interviews earlier this year that I seemed to get along just fine with tenured faculty; however, there was clearly some tension between myself and the "younger" tenure-track faculty at several places that I interviewed. If age did indeed play a role in this perceived tension, one may easily see why younger candidates would have an advantage. Furthermore, since most tenure-track candidates are from the same age group, the fact that an "older" candidate doesn't gel with his peers of the same academic rank for "some reason" makes you stick out like a sore thumb.
Ultimately, I think that if age does play a discriminatory role in faculty hiring decisions, it may be that it is an indirect discrimination. That is, in a direct sense, nobody cares about your age or have been trained to think that way. But if you don't have similar personality characteristics of those tenure-track faculty already in the institution's employ, then there is still some likelihood that you will be marked as a bad fit for the job. How high that likelihood is I cannot say.