Train up a child
Whether the proverb promises a guaranteed lifelong outcome or states a general principle of parental training that admits exceptions.
Proverbs 22:6
6Train up a child in the way he should go, and when he is old he will not depart from it.
What each tradition says
Rabbinic / Midrashic
Rashi states that according to what a child is taught and trained in, whether for good or bad, he will not turn away from it. Even when he grows old, it will remain with him.
Rashi (trans. A. J. Rosenberg), Rashi on Proverbs (The Judaica Press complete Tanach with Rashi) - Sefaria — Comment on Proverbs 22:6
Patristic
No sourced position found. Checked Clement of Alexandria, The Instructor, Book III (newadvent.org/fathers/02093.htm): no treatment of Prov. xxii. 6; John Chrysostom, Hom. XXVII on John (newadvent.org/fathers/240127.htm): none; no patristic exposition of Proverbs 22:6 surfaced in the ANF/NPNF material on newadvent.org.
Catholic
Haydock, citing Jerome and Calmet, treats Proverbs 22:6 as a proverb, added by Jerome to make the sentence more striking. The child is compared to a tender plant, wax, or a new vessel, so early training leaves a lasting impression. Parents should proportion lessons to the child's capacity and make him relish them.
George Haydock (citing Jerome, Calmet), George Haydock's Catholic Bible Commentary — Comment on Proverbs 22:6
Orthodox
No sourced position found. Checked Philaret's Longer Catechism (pravoslavieto.com): no passage commentary on Proverbs; NPNF ser. 2 vol. 9 (John of Damascus, ccel.org cache): Proverbs xxii. is not treated; searched archive.org for a public-domain English commentary on Proverbs by an Eastern writer - none found (the Greek OT commentaries are modern translations).
Reformed / Protestant
Matthew Henry reads Proverbs 22:6 as holding out the great advantage of careful parental training: good impressions made in childhood ordinarily abide, so it is to be hoped children will not depart when grown or old. Yet he acknowledges exceptions—many trained in the good way have departed, including Solomon himself.
Matthew Henry, An Exposition of the Old and New Testament, Vol. III (Proverbs) — Comment on Prov. xxii. 6
Evangelical
John Wesley explains “depart” in Proverbs 22:6 as “not easily and ordinarily.” He thus reads the proverb as saying the child will not ordinarily or easily depart.
John Wesley, Wesley's Notes on the Bible, Proverbs (Wesley Center Online) — Chapter XXII, note on v. 6 (Depart)