Is this perhaps because older workers gravitate towards sane hours? Maybe because they develop interests, lives, families, etc outside of work that they commit to (moreso than most 20-somethings)?
As a side note: I'm over 40 now, but I do recall being 27 and interviewing someone in their early 40s who was qualified and enthusiastic. I passed on them in part because I felt guilty hiring them; to work on my team the person would have to move their family to a new city, and I didn't want the karma of bringing that many people into a world where the team/project I was on was staffed and led by 20-somethings -- and therefore chaotic and unlikely to survive long in any particular form.
As a side note: I'm over 40 now, but I do recall being 27 and interviewing someone in their early 40s who was qualified and enthusiastic. I passed on them in part because I felt guilty hiring them; to work on my team the person would have to move their family to a new city, and I didn't want the karma of bringing that many people into a world where the team/project I was on was staffed and led by 20-somethings -- and therefore chaotic and unlikely to survive long in any particular form.