Starting at age 6, every boy was educated in the books of the Torah (the Pentateuch). School was most likely held in the local synagogue and taught by the local rabbi. At the end of this four-year or so stage, the students would generally know the entire first five books of the Bible by heart.
The students who showed promise then went on to the next stage, while the other ten-year olds went home to learn the family business as an apprentice. By age thirteen or fourteen, these students would have the entire 39 books of the (Jewish) Bible memorized.
Only the best students would continue from here, applying to the rabbi to become one of his disciples. Only those the rabbi deemed capable of becoming like himself were accepted. The others typically went home to learn the family business.
As a disciple, the student would stick to his rabbi like gum to a shoe, leaving his family and friends and home.
When Jesus went and collected his disciples, he collected students who had dropped out or had been rejected and sent back home to learn the family business, like James and John, learning the fishing trade from their father Zebedee. Jesus later tells them, "You did not choose me; I chose you."
These boys may have been 14 or 20; we just don't know.
As Rob says,
Jesus took some boys who didn't make the cut and changed the course of human history.