Age discrimination is, as you noted, incredibly difficult to prove, or even to get to a hearing. Discrimination cases in general are fairly "lucrative" for lawyers, since most forms of discrimination permit "treble damages" as punitive compensation to the injured. The punitive damages can go almost entirely to legal counsel, so some lawyers are willing to take a case on "contingency."
The US Federal statutes against age discrimination DO NOT PERMIT a claim for punitive damages, so the most that can be claimed is "actual loss" which leaves nothing for paying the attorneys. It's extremely difficult to even find an attorney willing to discuss an age discrimination claim, and can be incredibly expensive to the victim to pursue one.
If there happens to be an applicable state law conditions might be more favorable; but I don't know of a state that has an age-discrimination statute more favorable to the victim than the Federal law.