Outside of some of the mentioned reasons (20-somethings are cheaper, ageism, etc.), I think there's a lot of emphasis on staying up-to-date. In order to be "desirable" you need to consistently be learning about new technologies, languages, frameworks, etc. ON YOUR OWN TIME. As you get into age 30+, many people are married, have kids, have enough money to travel, etc. thus you probably aren't going to be as interested in reading programming blogs and watching tutorials.