what to do about age discrimination from potential employers?

"Potential" is the key word. ---Nothing. Employers have every right to hire who they see fit. There is no requirement that they hire an old person or anyone they don't want, don't think is able to do the job, or for any other reason. Age discrimination occurs when the person is ALREADY employed and has been for many years, has a great history with the company and is then fired because of his age. That is the age discrimination that has legal recourse. You sue.

PROVE IT. Except you CAN'T.

Depends. Age discrimination applies to those 40 and over. So if you can prove you're not getting hired because you're 40 and over, you might have a case.

If you are under 40, you have nothing. Zilch. Nada.

Noting.
The courts have pretty much eliminated age discrimination as a viable course of legal action. Old people are not a protected class.

Employers are not suppose to ask your age on questionnaires nor are they to discriminate against age.