Terence Tao went there.
Around 15 of the Field Medals winners were participants in IMO, including people like Grigori Perelman or Peter Scholze, along with probably some other interesting individuals (like Eric Lander, for example).
Some of you guys are making a stupid point here where everything in math is just the result of "mathematical geniusness" when it's quite more complicated than that. Also, it is an important point to make that obviously going to IMO is not a requirement to become a great mathematician, in fact, that's probably the reason why Asia has not produced many perfect proficient scientists as they have a more "crack the test" approach (study 12 hours a day not expanding your knowledge, but drilling what you know until it's basically rote memorization, do thousands of previous tests, etc). But the IMO is the best way to introduce mathematical maturity and abstract thinking to children and teenagers, and I think one of the biggest factors in success as a mathematician is becoming engaged in the subject as early as possible. So it's not a 100% likely, but it's pretty fucking close unless you are born in France, in which case shit is different (CPGE).
But anyway, seeing how education is becoming more and more of a competition and how these Olympiads are also becoming more popular, it wouldn't surprise me if later on most, if not all mathematicians, will come from there.
