The plain reading of the pasuk, cited by Rashi (Mishlei 22:6) and Metzudat David (Mishlei 22:6), is that whatever one impresses upon a child in his youth — for good or for bad — will remain with him even in old age, since the habituation of childhood sets a lasting imprint.
Malbim (Mishlei 22:6) reads two distinct commands in the pasuk: first, that a child must be trained early because habits formed in youth create a permanent impression on the soul in a way that adult training cannot, and second, that the training itself must be suited to the particular nature of each child, since every child is constitutionally disposed toward different intellectual and practical paths, and education must be fitted to the inclination and aptitude the child himself displays.
Ralbag (Mishlei 22:6) similarly explains "al pi darko" as referring to the path natural to the child's own temperament and character, teaching that once a child absorbs an orientation in his formative mindset, it becomes nearly impossible to remove later, whether for good or for bad.
The Vilna Gaon (Beur HaGra on Mishlei 22) grounds this in the idea that a person cannot break the "mazal" or innate nature he is born with, but free choice allows him to direct that nature toward righteousness, mediocrity, or wickedness; he reads "al pi darko" as meaning one must train the child according to the direction of his innate nature and disposition so that his natural tendencies are channeled into mitzvot, and this channeling will remain with him into old age.
Midrash Mishlei (22:3) records a dispute between Rabbi Eliezer and Rabbi Yehoshua on the pasuk, with Rabbi Eliezer holding that training a child in Torah while he is young ensures he will continue to grow in it, citing the continuation of the pasuk that even in old age he will not depart from it.
Sefer Chasidim 308 applies the pasuk practically to pedagogic method, ruling that a teacher must separate a sharper student from others who learn at a different pace, and must gauge whether a given child is suited to Scripture or to Talmud, teaching each according to where he shows success rather than forcing a uniform curriculum.
Sefer Chasidim 304 extends the idea of tailored, age-appropriate chinuch to content, teaching that a child must have a teacher who himself understands what he is teaching, and that matters of yirat shamayim and the reality of reward and punishment should be introduced to a child specifically because a child's heart accepts what he is told as true, before peers can influence him toward bad things.
Rambam (Hilchot Deot 1:7) and Rambam (Hilchot Deot 1:4) describe the broader process of character formation that underlies such training: a person habituates himself to the proper middah by repeating the corresponding actions again and again until they become easy and the trait becomes fixed in the soul, aiming always at the middle path between extremes.
The chiyuv of a father to train his son is also cast halachically in terms of active instruction in practical skills and Torah: Kiddushin 30b and Kiddushin 29a enumerate the father's obligations to his son — circumcision, redemption, teaching Torah, arranging marriage, and teaching a trade — deriving the trade obligation from the verse "Enjoy life with the wife whom you love," and Sukkah 42a describes the obligation to train a minor progressively in mitzvot, from lulav to tzitzit to tefillin to Torah, as soon as he demonstrates the relevant capacity.
Pirkei Avot 4:20 draws a related lesson about the advantage of teaching the young, comparing a child's learning to ink written on fresh paper that holds its mark, in contrast to teaching an old person, which is like ink written on erased paper.
Later moral and chassidic literature draws lessons of educational timing and approach from the pasuk: Even Sheleimah 6:6 insists that one must train children in learning for its own sake from childhood specifically because doing so forestalls the yetzer hara's later grip, invoking the pasuk directly to reject the view that a young child need not be trained seriously.
Rav Samson Raphael Hirsch frames true chinuch as bringing the young close to Torah and the realities of life from the outset, rather than filling them with fantasy, so that this closeness to truth accompanies them throughout their lives and remains with them even in old age.
Chochma uMusar (1 p. 149) is read not quotably but informs us that it situates chinuch within faith formation, teaching that accepting mitzvot through faith (as exemplified by Avraham) yields reward beyond mere performance, and warning that even a single false promise to a child leaves a lasting impression — a caution directly relevant to the integrity required in training a child "al pi darko."
Among the page-scan sources, Nitei Gavriel (Upsherin, perek 17) reads the pasuk as calling for training a child in prayer, practical mitzvot, and simple emunah in the one God of Israel, while Beit Yaakov (Shmini 50), Beit Yaakov (Shmini 51), and Beit Yaakov (Shmini 49) each explain chinuch as habituating a person to something not previously practiced, such that the habit remains even in old age and enables achievements otherwise beyond his capacity, and Chochma uMusar (1 p. 250) raises the question of how such permanence in the path of one's youth can itself be considered a mark of success.
Tzitz Eliezer (21:32) brings the comparison, in the name of Yonah, between the pasuk and the idea that children are like saplings nurtured in their youth: a young plant can still be shaped into a straight tree, whereas correcting a tree once it has already grown crooked is far more difficult, underscoring the urgency of early, direction-appropriate chinuch.
