Perfect Youth and Infancy

In fact, one should even correct wrongdoings that preceded their Bar/Bas Mitzvah (as the Alter Rebbe states in his Shulchan Aruch). Indeed, we find that many Sages made an effort to correct "wrongdoings" that had taken place while they did not yet have sufficient understanding to realize what they were doing – including when they were tiny babies; from the moment they entered this world.

In his Shulchan Aruch (ibid.), the Alter Rebbe states regarding sins that were performed when one was a minor: Although he need not do Teshuvah for them now that he is an adult, nevertheless, it would be appropriate to accept upon oneself something, at least, in the way of Teshuvah, to gain atonement. (Parshas Behar 5749, ibid.)