I believe it is very bad, although I do not have data. I believe that if a 46-year-old, with an excellent vita and newly minted Ph.D in hand, applied for academic economics jobs at the top fifty research universities, the individual would receive very few “bites.” Unless of course he or she managed to cover up his or her age. (I am very pleased with the openness of my own university, I will add in passing.)
Perhaps there are not many examples of this kind of age discrimination (do you know of any?). In part that is because older individuals are so discouraged from going down that path in the first place. Furthermore it is likely harder for older individuals to go down that path. In addition to life-cycle considerations, there may be age discrimination at the stage of graduate admissions.
I rarely hear complaints about age discrimination in academia, though I often hear complaints about gender and race discrimination. I believe all of these phenomena are real (and unfortunate), and I wonder what exactly this discrepancy indicates. If anything, I suspect age discrimination is far more extreme, at least when it comes to the final stage of the process, namely the actual interview and hiring decisions.