I think that some people think people should start school at the normal age instead of older because they claim they might become too old to learn. I learned from https://www.youtube.com/watch?v=n7jHT7Tumcw that in China, students start learning fractions at a very young age and then end up learning them quite well and quickly. According to https://www.inc.com/bill-murphy-jr/science-says-were-sending-our-kids-to-school-much-too-early-and-that-can-hurt-th.html, it's better to start school at an older age. I think they mean it's better to start grade 1 at an older age because of course those who don't go to kindergarten then start grade 1 at the normal age start an an older age than those who do go to kindergarten then start grade 1 a year later than normal. Maybe for some things people can become too old to learn. For example, Mary and Janelle from Sister Wives were raised very differently and then they had trouble understanding each other's way of thinking until they went to a therapist to understand each other. That doesn't mean there's a universal tendency for everything to be harder to learn when you're older. Maybe the real reason why students in China learn fractions so quickly and well is not because they're taught so young but either because of the method they're taught fractions as described in the video or because there's a general tendency for Chinese people to be more likely to be smart because of their genome.
Maybe Mary and Janelle being too old to learn each other's way of thinking actually meant they were very smart. They cannot possibly figure out absolutely everything anyone else can figure out like a God and maybe being too old to learn means they're smart enough to be quite well aware that although there are risks with doing things exactly the way they are used to, there are also risks with trying doing things in a new and different way.
I think that in addition to that, sometimes when somebody learns something while they're so young, they're more likely to misinterpret what they're getting taught. This answer appears to support my claim. According to https://matheducators.stackexchange.com/questions/926/whence-the-everything-is-linear-phenomenon-and-what-can-we-do-about-it, maybe some students learn something wrong and then after unlearn it later. Also in one episode of "My 600 lb life" the adult patient who is overweight during the episode, at the time she was 5, her parents kept saying "You're too fat. You can't have it" meaning dessert and then she thought it was a punishment for being fat and got very depressed. Unconsciously, that's probably because it's possible to deduce from the fact that she got a punishment that she did something wrong. I think that actually she was too young to be able to figure out how to interpret the message the way she was intended to and an older person who's fat if that was said to them would be able to figure out that the parents weren't trying to be so mean and meant they should not have dessert because it's very sweet and might make them fatter and then they might die of a heart attack.
Is it actually true that students tend to learn better from school when they start grade 1 at an older age? If so, could one reason be because those who start grade 1 at the normal age are more likely to face the problem where they are not yet ready to learn the basics and then they move on from the basics before they were ready to be able to learn them and then struggle to learn the later material because they're missing the basics? Could another possible reason be because they're less likely to be young enough to misinterpret what they're getting taught?