וַיֹּ֨אמֶר שָׁא֜וּל אֶל־דָּוִ֗ד לֹ֤א תוּכַל֙ לָלֶ֙כֶת֙ אֶל־הַפְּלִשְׁתִּ֣י הַזֶּ֔ה לְהִלָּחֵ֖ם עִמּ֑וֹ כִּֽי־נַ֣עַר אַ֔תָּה וְה֛וּא אִ֥ישׁ מִלְחָמָ֖ה מִנְּעֻרָֽיו׃ (ס)
And Saul said to David: ‘Thou art not able to go against this Philistine to fight with him; for thou art but a youth, and he a man of war from his youth.’
Kitzur Ba'al HaTurim
מנעריו From the youth in 3 different places in Tanakh teaches "a man of war from his youth (I Sam. 17:33)." "Mo᾽av has been at ease from his youth (Jer 48:11)." From his youth, a man must fight with the evil inclination and if he overcomes him and his victory is that he is Moab from his youth, "and he has settled on his lees (Jer 48:11)."