We don't work a 9-10 hour day. I've written up the whole thing here if you're curious: http://ryanleecarson.tumblr.com/post/21708810513/4-day-week
If you want to reduce the total hours per week, doesn't it make more sense financially to work fewer hours per day but for five days and be able to pick up your kids at an earlier time each day? I would think that would be the case for most family folk. Don't your kids get off school or child care at somewhere around 3pm? Who takes care of them between 3pm and 6pm and how much does it cost you?
Finally, what a family person wants differs from what a single 20-something year old wants. 9am works great for a family man. 9am does not work fine for a 20-something year old, but in the majority of companies the family man is the boss and dictates the 9am start time.
It's interesting how you think a 20-year old can't fit into the world's normal working hours. I managed just fine working normal hours when I was in my 20s.
Regarding younger folks, most people work regular work hours in their 20s, but if you take a poll, I bet plenty of them would prefer other arrangements. In a city like New York, there's plenty to do between the hours of midnight and 4am any night of the week, regardless of one's view of such activities. To paraphrase one founder from a previous job, "it's fun going out Sunday nights. You get to meet the people who don't have a 9-to-5 job." If you think back to college, where people could more or less choose their daytime schedule, most students did not go to sleep before 1am. Being stuck signing up for the 9am class was a major disappointment for people. At the same time, though, lining up your schedule to have every Friday off (which was a rare possibility) was highly prized!
My generalization is that people in their 20s favor flexibility to accomodate nightlife (and fun in general), whereas family folk favor a set reliable schedule structured around their kid's schedule and maximizing time with the significant other.
Bottom line though, employers should sit down with their hires on the first day and ask them what's important to them in their personal life so it can be accommodated.
Explain why 9am does not work for a 20-something. Yes, I am a family guy as well, but even when I was single, I was at work by 9am almost every day at the ripe old age of 21. If you said your reason was, "I don't like waking up before 9am," I'll buy that, but the position you state is an empty one at best without some sort of evidence.
At least in school you get a whole summer to recover.
As one of Helidor's other posts mentioned, it needs to be mentioned that a flexible schedule/later "clock-in" time is acceptable as long as you get your work done if that's what an individual truly cares about. This all comes back to the position that I and most others I know hold, and that being that you pick your values that you won't budge on, make them known, and poke around until you find something that fits.
To your point of being forced to do something you don't want to do, having been in a company that "forced" me to work 60+ weeks on a regular basis and having almost no time to spend with my family drove me up the wall. I bounced out as soon as I found something better. That presented a very big challenge for all three of us in my family to the point where my kid wondered if I was going to be home in time to put her to bed some days. Hearing those kinds of statements from your kid really puts things into perspective and makes you see if what you are doing is really worth it.