As for kids, I agree with you that the majority of them can potentially be good at almost every field. But as for adults, I’m not so sure. Most of us become way too rigid as we age, learning capability and more importantly motivation becomes way lowered. So someone who had trouble with basic algebra (at the time mostly due to a bad teacher/experience with math) will not generally be able to overcome his/her identity of “being bad at math”. Of course, there are exceptions.