Earlier quoted context omitted.
This refers to a category of person protected by the Civil Rights Act of 1964[1], Title VII, as extended by other laws over time. It "prohibits discrimination by covered employers on the basis of race, color, religion, sex or national origin". Those of age 40+ years are covered specifically by the "Age Discrimination in Employment Act of 1967". In other words, "age of 40 or more years" is a "protected class". [1]: ht…
I'm curious as to whether these laws make a positive difference in practice. If someone is truly racist/sexist/ageist then they could probably invent a lawful excuse for their hiring behaviour anyway. On the other hand, if they are rational then (a) they would already benefit from hiring these workers cheaply because the demand is lower, so they would win in the market, and (b) the discrimination laws would increase…
I think they have, based on reading through a dozen or two discrimination court cases, the EEOC guidelines (the EEOC "is a federal agency that administers and enforces civil rights laws against workplace discrimination", quoting Wikipedia), and comparing historical accounts of the workplace environment to the modern one. (For example, the detailed EEOC policy guide on sexual harassment is at https://www.eeoc.gov/policy/docs/currentissues.html . See https://www.eeoc.gov/laws/types/age.cfm for an overview of the age related issues.) Plus many offices have EEOC training so people are aware that these laws and restrictions exists.
Your point about finding a "lawful excuse" is a bit of an oxymoron. If it's lawful then it's not a violation of the relevant civil rights laws, so it's not really an excuse.
People do use an excuse which they think is legal, and would be legal if the person affected weren't part of a protected class. (For example, Disney had a costume policy that employees were not allowed to have a beard. Sikhs wear a beard as a reminder of their commitment to their faith. This gives a protected status that does not apply to most beard-wearing people. See http://money.cnn.com/2015/07/10/news/sikh-turban-beard-disne... .)
However, most employers don't really know the relevant laws, and go more on what they think is justifiable rather than what is actually legal. That's why there are successful lawsuits.
Regarding rationality, people simply are not 100% rational. The last decade or two of behavioral psychology has convinced me of that. If people are rational, then why do you think discrimination exists at all?
Otherwise, if you don't believe that discrimination exists, then I think it's pointless to continue any conversation.
You ask for empirical data. That's of course a hard thing for many laws, with gun control laws, minimum wage laws, and education policies like charter schools being three of the more notoriously debated.
My experience when I see people ask for empirical data is that no matter what evidence I can provide, it's insufficient. That is, they use it as a way to externalize their doubts, without saying what level of data would overcome their objections.
I'm not saying you are like this, I've just been burned too many times by others doing this. So before I look for empirical evidence on this topic, can you point to other social policies you think there is sufficient empirical data to justify them? What would valid empirical data look like to you?