C. F. Keil and Franz Delitzsch on Jeremiah 1:6

Jer 1:6

The divine call throws Jeremiah into terror. Knowing well his too great weakness for such an office, he exclaims: Ah, Lord Jahveh! I know not how to speak; for I am נער, i.e., young and inexperienced; cf. 1Ki 3:7. This excuse shows that לא יד means something else than לא אישׁ דברים, by which Moses sought to repel God’s summons. Moses was not ready of speech, he lacked the gift of utterance; Jeremiah, on the other hand, only thinks himself not yet equal to the task by reason of his youth and want of experience.