When do we instruct a boy to shorten his garment above the ankles?

The child is commanded to attend prayers with the congregation, to use the siwak, and to perform other religious duties after seven years of age. They are also to be taught the prohibitions of zina (unlawful sexual intercourse), sodomy, alcohol, lying, backbiting, and similar acts. Parents are obligated to teach their children ritual purity (tahara), prayer, and religious laws after seven years of age, and to discipline them for abandoning these after ten years of age.