The books are recommending that students intentionally delay a year and don't start school on time? Feel free to share any sources, as it's not believable as written. If that's what people want to will themselves to believe - so be it - but it still doesn't make it accurate. If you want to not go by the state schooling schedule, you need to go private, or convince a school district that your kid is slow enough to need special attention and isn't able to start school as recommended - which comes with its own complications.
Experts may certainly be recommending that starting school early, and trying to graduate sooner than expected, while once considered something to shoot for, may no longer be as helpful as it once was. For one thing, the removal a few years ago of most standardized testing for college admissions means that more weight can be put on grades/activities than a single test score; an extra year of schooling can certainly help the former more than the latter. Now that the pendulum is swinging back to requiring such standardized testing more and more, it remains to be seen whether it will go back to being advantageous to skip grades vs. staying on schedule.
At some point perhaps the public school system will be challenged in numbers by the students in whatever is considered "private" after a few more years of people placed in charge of public education to explicitly weaken/disable it to the point such that private becomes the reasonable option for many more than today. Right now about 10% of kids in the US are in private. It also varies widely by state - with Wyoming having 97% in public, while only 45% are in public in DC. (
link) If those numbers change markedly, it makes it less useful for any other context (like soccer organizations) to make assumptions for translating what grade a kid is in to what age they actually are. But we are still very far from that being the case.