They're probably not being intentionally ageist. My guess is that the differences in sense of humor, lack of desire to hang out with coworkers, and other social

They're probably not being intentionally ageist. My guess is that the differences in sense of humor, lack of desire to hang out with coworkers, and other social differences just make them seem like unappealing "culture" fits. And it's true, if you're just hiring for people you can also hang out with and you're in your 20's, of course you won't like them. At some point you get the revelation that people have their own lives and there is a social relationship along the lines of "we work together but I'm otherwise not interested in you," and that this is actually a very common and viable type of social relationship.