It's extensive and well-settled law and precedent. Google has all the details.

Summary:

Federal law (not the constitution) prevents age discrimination.

Fed can implement other laws which *do* allow discrimination, since it's not a constitutional thing, prime example is age 65.

Fed/state/local governments can set age limits for public safety jobs, where they can demonstrate a legit basis for that. Private employers cannot do that directly, but they might employ people who require a government license with an age limit (like us).

There is a very specific carve-out for private employers to set an age limit of 65 for bona fide executives. The law proscribes the creative use of the "executive" label, ie they can't call you the COO of Flight 123, or the FO the VP of landing gear and comms to justify mandatory early-out.

This is for the upper end, discrimination is obviously allowed for youth, for their own safety and well-being, and for maturity & judgement reasons (ATP, law enforcement, military, etc).