A.T. Robertson Commentary Hebrews 5:13

"For every one that partaketh of milk is without experience of the word of righteousness; for he is a babe." — Hebrews 5:13 (ASV)

Without experience (απειρος). Old adjective (alpha privative and πειρα, trial). Inexperienced. The babe (νηπιος, old word, negative νη and επος, word like Latin infans, infant, not able to talk), not able to chew if one uses only milk and is without teeth. Perhaps moral truth is meant by "word of righteousness" (cf. 1:2; 2:3 for the word spoken by Christ).