Once again, "culture fit" against diversity:

"Twenty years ago, when venture capitalists invested in young founders, they usually insisted that founders team up with older, seasoned executives to provide “adult supervision.” Lately the conventional wisdom has been that it’s better to let young founders go it alone. The consequences have been predictably disastrous. Young male founders hire young male employees, and spend huge money building kooky office frat houses. "

It's still illegal to discriminate hiring based on age right?

Making hiring and firing decisions based on age is illegal, but age discrimination is rampant in the tech industry, and everyone knows it, and everyone seems to accept it. What other industry operates like this? What would the world be like if doctors, lawyers, or airline pilots — or anyone, really, other than professional athletes — had to accept the idea that their career would end at age 40, or 50?

One excuse for pushing out older workers is that technology changes so fast that older people simply can’t keep up. Veteran coders don’t know the latest programming languages, but young ones do. This is bunk. There’s no reason why a 50-year-old engineer can’t learn a new programming language. And frankly, most coding work isn’t rocket science.

Yes, it is, but it is also very easy to NOT hire a person you do not want to hire for the reasons having nothing to do with the age, and it NOT easy to prove that you did not get this position because of your age. "Not a culture fit" is a perfectly good explanation.

Right, I've heard that excuse many times. 

It's fascinating how overtly ageist some of these companies can be.