Are you saying that starting Kindergarten one year late is the reason they were getting out of school later and putting off pregnancy? I find it hard to believe that the one extra year made that much of a difference. I definitely agree with your premise - that people are putting off children longer but I really don't think it has anything to do with redshirting.
Yes, I am the PP that made the argument about graduating college early to jumpstart your career. While I agree that a year probably doesn't mean much in the scheme of things, it seems equally ridiculous in the long run to delay kindergarten by a year to provide advantages Malcolm Gladwell's outliers-style as it does to skip people ahead only for the sake of jumpstarting their career. I'd almost say that being able to graduate college early provides even more advantages, because you feel less of a squeeze to get your adult life going if you are a year or two ahead given how many women in their late 20s and 30s worry about their biological clock. Keep in mind that any academic advantage due to redshirting has been shown to even out by 8th grade, anyway.
I find it ironic that people complain about how young people are delaying their careers, marriage, moving out of their parents house, etc. while at the same time parents are purposefully holding kids back at many different levels. People start their kids off in kindergarden a year late, encourage gap years between high school and college, and so on and so forth. While on an individual basis, it might be beneficial for a small minority of children to start kindergarden late, the trend to hold kids back with birthdays as early as April seems to be ridiculous. Children have a lot more resilience than people think, and having a year of struggle because you are slightly developmentally behind a few peers is probably not going to cause lasting damage.