I want to reinforce what @RichInPitt said.
Through my son, I know dozens of kids that made USAMO, and about 80% of them ended up attending an HYPSM or CalTech. Among them, it was rare to have someone who went beyond Calculus by junior year. These kids could have learned Calculus by sixth grade if they put their mind to it. But they were correctly guided that there was no advantage in doing so.