Legal discrimination can occur in explicit, discrete instances, such as your example about this particular man. Illegal discrimination can, too. But illegal discrimination can also occur in a systemic, implicit way.
Systemic bias exists. One manifestation of that in this industry is the signaling in certain job descriptions and hiring practices. We needn't point to explicit, specific examples (e.g. "hiring manager told me point-blank 'sorry, we do not hire people over 30'") to see a broader effect.
Offering below-market salaries for 80 hour work weeks isn't illegal age discrimination even if this highly correlates with age, those are legitimate criteria that affect work performance, and thus are valid; just as criteria that require workers to be able to lift a certain weight is a legitimate criteria if appropriate to that job role, even if it would disqualify almost all women for applying.
Disqualifying an old man because you think that people of his age won't work 80 hour weeks for a poor wage or can't carry the required goods is prejudice and illegal discrimination. However, disqualifying an old man because you know that this particular man won't work 80 hour weeks for a poor wage and can't carry the required goods is fair game, it's a reasonable evaluation of a potential employee.