They grew up with exposure, context, experiences and knowledge of those things mathematics is designed to describe. Our oldest son, Kirby, worked in a games store from the time he was fourteen, and was running tournaments for Pokemon, Magic the Gathering and other such structured strategy games, in the store and at hotels in town for several years. The knowledge required to play those games and even more to organize, judge and score tournaments, is huge.
When Kirby was 18 he took his first math class, at the community college. Like a musician who can't read music, he was baffled at first, but once he understood the notation, he soared, and had the highest test score in the class.
To some people reading this, it might seem there was no "higher math," but what we have done is create a home in which algebraic thinking is a standard part of conversations. Our interactions are analytical and involve factors and projections. They see the concepts and they use them.
(There's a link there to the published German version.)
photo by Belinda Dutch