The Birth of the Good Inclination
In rabbinic texts, the distinction between childhood and young adulthood is the birth of the yetzer hatov, the good inclination.
The rabbinic duality of yetzer hara, the so-called "evil inclination," and yetzer hatov, the "good inclination," is more subtle than the names connote. Yetzer hara is not a demonic force that pushes a person to do evil, but rather a drive toward pleasure or property or security, which if left unlimited, can lead to evil (cf. Genesis Rabbah 9:7). When properly controlled by the yetzer hatov, the yetzer hara leads to many socially desirable results, including marriage, business, and community.
For the rabbis, adults are distinguished from children by the yetzer hatov, which controls and channels the drives that exist unchecked in the child. Thus children may seek pleasure and acquisition, but they are not able to create a sanctified relationship or exercise the responsibility to engage in business.
Developing a Moral Sense
The classical text that describes how children are born with a yetzer hara but only later develop a yetzer hatov comes from Avot d'Rabbi Natan, a third-century midrashic companion volume to the more well known Ethics of the Fathers.
The yetzer hara is 13 years older than the yetzer hatov. While still in the mother's womb, the yetzer hara begins to develop in a person. If he begins to violate the Sabbath, nothing stops him. If he commits murder, nothing stops him. If he goes off to another sin, nothing stops him.
But 13 years later, the yetzer hatov is born. When he violates the Sabbath, it rebukes him, "Airhead [literally: "empty one"]! Don't you know it says 'Everyone who violates it will surely be put to death' (Exodus 31:14)?" If he is about to commit murder, it rebukes him, "Airhead! Don't you know it says 'Whoever sheds a man's blood, by man will his blood be shed' (Genesis 9:6)?" If he is about to engage in a sexual sin, it rebukes him, "Airhead! Don't you know it says 'Both the adulterer and the adulteress will surely be put to death' (Leviticus 20:10)?" (Avot d'Rabbi Natan 16).
The examples in this text are extreme, but serve to make the point that even laws punishable by death are not necessarily "self-evident" to children. The young adult is not described as someone who has developed a sophisticated moral sense; in fact, the early adolescent may base moral decisions entirely on fear of punishment. Yet by age 13, the child's moral sense has developed sufficiently to hold the child responsible for his or her actions.
The distinction between the yetzer hara and the yetzer hatov is also derived by the midrash and later commentators from the verse, "Better a poor and wise child than an old and foolish king who no longer knows to receive admonition" (Ecclesiastes 4:13). The 11th-century commentator Rashi interprets this verse phrase by phrase:
Better a poor and wise child: This is the yetzer hatov, and why is it called a child? Because it does not enter a person until 13 years.