Many decades of cognitive science disagree with you.
See Piaget for one of the earlier researchers on this topic. Even though his original ideas have been refuted or refined, the basic premises of his ideas are still prominent among the large group of modern researchers that some people dub neo-Piagetians.
Even here in America, tutoring schools such as Russian Math or Kumon are successfully teaching advanced topics to children at much younger ages than the public schools. I know the temptation is to just say its its high aptitude kids that are being sent to these after school programs, but it's not. It is just rich kids whose parents want to give their children a leg up. Almost all children could learn the topics, but we won't put in the effort
Based on your reply, I’m fairly certain that you are not familiar with the research and your not familiar with what is actually happening in Eastern Europe, Kumon, etc. in relation to that research. As a professional in this area of research, I feel fairly qualified to say that I am.
I am happy to agree to disagree about what the problem is here, but I strongly encourage you to look deeper into what is actually happening in the research and the learning environments you are praising/promoting before you propagate a loose collection of anecdotes as data that points to a clear solution.
Just because I completely reject the ideas you may be involved with researching doesn't mean I am ignorant of them, I just haven't been convinced that they are based in reality.
Interestingly, on this we agree. That’s why throwing more bodies at the problem (your initial suggestion) is not a solution that I think would work. When a typical educator is asked why a certain teaching or learning method works, their answers tend to be both incorrect/incomplete and remarkably shallow (e.g., it worked for them when they were in school).
The research available points at multiple tried and true ways of learning, but getting these ways of learning implemented in households and in the public school system in the US is a Sisyphean task.
Most of the broader ideas that he espoused and is famous for were refined in a way that made them accurately generalizable. Typical research timeline where an idea is introduced and is refined over time.
Some of the specific ideas he stated (and typically backed with data) was not generalizable to broader populations and was therefore refuted as an absolute. The data and conclusions were not necessarily wrong for the sample of the population studied, but they were not found to be as broadly true as initially believed.
This additional research led to the refinement of his theories, to great effect, imho.