Companies with more than 20 employees cannot discriminate against people who are 40 or more years old.
This law is becoming a real hot button since the Baby Boomers, a fairly large percentage of our population, are over 40. And, within the startup world, there really are clear age biases, starting with the VCs who believe that the best company to invest in is one founded by someone under the age of 30. And since most people tend to hire people like themselves...
There are a bunch of ways to run afoul of this law, and the two most common occur in hiring practices and as part of a layoff. Usually, these occur in a subtle way, in a way where you don't think you're discriminating.
Here's the way these two usually play out.
Hiring: Wow, look at this guy's resume. He's got exactly the skills we need, plus a bunch more. He's been in the industry since 1985, and has done some great work at companies that have done great things. But, this is a mid-level SDE role, and I'm sure he's too expensive.
There are a number of key words here: "skills", "1985", "I'm sure". The "I'm sure" is a key giveaway. If you haven't stated the salary range in the job ad, and/or if you have not asked the candidate what their desired salary is, you can't say "I'm sure". It's very possible that you might get a great candidate that others have passed over because of their (poor, misinformed, illegal) assumptions. Too bad for them!
Layoffs: We have to seriously reduce our burn rate, and we need to reduce payroll expenses. I've done a stack-ranking of our most expensive people, and suggest we start by getting rid of them first.
This example is on the simple side, but if you have a spreadsheet of employees that happens to contain salary info in it along with other data like title, department, etc, and you're using it as part of your layoff decision process, you might be on thin ice. The reason is simple: Older people tend to make more money than younger people in a given role. So, while you may be operating with the best intentions to reduce expenditures by the maximum amount, you may also have inadvertently run smack into age discrimination.
Disclaimer: I am not a lawyer. U.S. laws are subject to change. States may or may not have laws on the books that counter or add to our National laws. This is written as-is. If you have questions, go see your lawyer!