Likely yes: https://www.dol.gov/general/topic/discrimination/agedisc (second paragraph)

I'm not sure what if means by "certain" applicants and employees, but my guess would be jobs where it's relevant, like physical labor, are excluded.

I would imagine it's extremely hard to prove though. The prospective employer could just maintain that the applicant "wouldn't be a good culture fit" or similar.

Disparate Impact laws mean that it's not always necessary to disprove a pretext.

How much do you want to gamble?

Retainers start at around $10k.