As long as you have a normal understanding ability (you can know what conditions the topic provides and what you need when you see it, and you won't misjudge it most of the time), anyone who can do general topics can learn Olympic Mathematics.
Olympic Mathematics attaches great importance to problem-solving methods and ideas. If you know the method, you will solve many problems. The idea is right, the problem is not difficult. Many problems in Olympic Mathematics are very representative, and sometimes a problem represents a method of solving problems.
To put it bluntly, learning olympiad is learning methods and ideas. Students with average grades don't have to follow the teacher. Parents can teach some simple things.
If a child wants to learn, you can create conditions with him to improve his math performance. As long as you work hard, you will always get something.