>But who gets to set that threshold? Who determines what is reasonable? What if that performance threshold can only be met by fit young people and only until their bodies can't take the stress anymore? How will workers even assess whether or not they can do the job until they try?

If it's harmful to the worker, then it should fall under the purview of OSHA in the US.

Otherwise, that threshold is set by consumers who choose to patronize businesses that treat their employees in a certain way. Almost all of the time, for goods that can't be distinguished from one another, consumers will choose to patronize a business offering the lowest price available to them, hence the businesses that squeeze the most out of the workers survive and the others do not.