Even a child is known (maketh himself known) by his doings. (For "even" (gam), see on Proverbs 17:26.) A child is open, simple, and straightforward in his actions; he has not the reserves and concealments which men practise, so you see by his conduct what his real character and disposition are.
Ewald takes מעלליו in the sense of "play," "games;" but it seems never to have this meaning, and there is no need to change the usual signification. The habits of a life are learned in early age. The boy is father of the man.
Delitzsch quotes the German proverbs, "What means to become a hook bends itself early," and "What means to become a thorn sharpens itself early;" and the Aramaean, "That which will become a gourd shows itself in the bud:" Whether his work be pure ("clean," as Proverbs 17:9 and Proverbs 16:2), and whether it be right.
His conduct will show thus much, end will help one to prognosticate the future. Septuagint (according to the Vatican), "In his pursuits ( ἐπιήδευμασιν) a young man will be fettered in company with a holy man, and his way will be straight," which seems to mean that a good man will restrain the reckless doings of a giddy youth, and will lead him into better courses.