Maybe, but that seems kind of crazy as I had tangible proof of age discrimination. It's not like they'd say someone who was 38 was too old and someone who was 42 would suddenly not.
More generally, discrimination protections for employment are much narrower than commonly assumed to be. Discrimination is legal by default and becomes illegal only when a specific law cites a specific protected class. Some characteristics that are perfectly legal to discriminate on because there is no US law protecting the class: age under 40, smoking, obesity, height, car you drive, method of commuting, sports team favoritism.