Imagine a child aged 11. They have just finished primary education and now moving into secondary education. This child has shown a great mathematical talent/disposition since a very young age. By the end of primary school they have completed studies that a top tier 13 year old would be doing (in my experience (UK) primary schools are better than secondary schools for giving stretching pieces of work like this).
How should the school/parents deal with this situation? The problem with learning with children their own age is that they are unlikely to learn anything new in class for two years, may get bored with the subject, and lose the enthusiasm they had for it.
One option is that the parents (or a tutor) could teach them extra maths outside of school. However the child may resent having to do extra learning, and will still be bored in class. The other factor is, what is the end-game in all of this? To do undergraduate maths at an earlier age? I'm not sure that's a good idea, nor that you should be pointing an 11-year-old down such a narrow path.