The biggest problem with age discrimination is that it's subtle-- not overt. It exists in the form of harsh age-grading for accomplishments, because the delta between the average and the maximum possible at a given age grows, so the average person's deficit compared to where they "should be" becomes more of an issue. No one would pass on a 55-year-old CS luminary who's still writing papers, but 40-year-olds with the careers expected of 35-year-olds are screwed. After a certain age, being merely above average isn't acceptable anymore.

Most companies would rather bet on a young 6 than an old 8, because they overestimate their ability to mold the 6 into a 9 (which is actually very hard to do).

That's why establishing age discrimination is impossible. You'd have to generate identically indicative/"qualified" resumes at very different ages, which is probably impossible to do.