I am about to start this textbook, one book that fills the needs of all students from age 12 to graduate college age 26. Of course, teachers will have to fill in blank pages and supplement with homework. My text is an overall text of all of mathematics.
I felt the need to write this because Old Math is so riddled with mistakes, that I stopped writing Correcting Math because the mistakes are so large and many, that there is barely anything remaining.
Now I pick the age of 12, because I can barely remember my ownself in Grade School at that age, where we need to start teaching True Math and where students are lead astray starting age 12.
Education should never ever be a situation in which you teach students one year and a few years later the new teacher says all that other stuff was rubbish and now you learn what is taught. I recall about three times in my education the teacher saying-- forgot what you learned in the past for it is rubbish.
That is why I feel age 12 is where we have to intercede with a text on math where you no longer hear-- forget what you learned in the past.