Madoosa, at age 5 that is fine. Sounds like it is a class rather than travel soccer (which I hope is not available at 6U, though I have heard of travel baseball & softball in CA at 6U).
The problem is when you get into travel or club sports. From a legal perspective, there is no issue in the US in many travel sports with a younger kid being on an older team. While these kids may think it will be more challenging, there is no challenge when you sit on the bench.
Hope the OP finds an appropriate math challenge for her son. Unfortunately, that will be many years in the future in school - there is another thread about that.
