People get the wrong impression that it's a SV thing. It's not just SV. Lots of employers (who are themselves age 40+) like to hire 20-somethings.
As one example, there was a reddit thread with a woman in Tennessee trying to start a hot sauce business. In one of her replies, she wrote: "I wanted my company to be young and hip, so I hired young and hip -- okay, ‘hip’ might be subjective, but we’re definitely young."[1]
It was fascinating to watch an honest moment of communication because she later deleted that reply. However, others had already quickly replied to it which left her original comment for prosperity.[1]
In another example, the aides that sit directly adjacent to the President's oval office are traditionally staffed by 20-somethings.[2] There's nothing in the job description that says the gatekeeping work can't be done by senior-citizens but every WH Chief of Staff always wants the high energy of a recent college grad stationed there.
It's not just programming. Whether it's packaging up food sauce in Tennessee or filtering visitors into the President's office in Washington DC, employers want the vibe of a young crew.
[1] original text still in a reply: https://www.reddit.com/r/pics/comments/4hiipf/update_hi_redd...
[2] Bush aides: https://youtu.be/lNhpsPvMYAE?t=5m37s
Obama aides: https://youtu.be/p-U0-0anzV0?t=28s
Clinton aides: https://youtu.be/uzZ-gfvXN-c?t=25m17s