What is the meaning of "Ahah"?
'Rashi': It is an expression of wailing.
Radak: One calls out so over pain and worry.
Does Yirmeyah not know how to speak?!
Radak: He does not know how to rebuke.
Why did he say "I am a Na'ar"?
Rashi: I am not proper to rebuke [Yisrael]. Moshe rebuked them close to his death. He was already esteemed in their eyes via many miracles that he did for them - he took them from Egypt, split the sea for them, brought down manna and Slav, gave to them Torah and the well. I, at the beginning of my mission, will I rebuke?!
Radak: Refer to 1:6:4:2.
Malbim: (a) A youth is not bold to approach elders [to rebuke them]. (b) He does not know how to arrange his words in nice, clear expressions. This requires great habituation! (c) The nation will not honor a youth [who rebukes them]. They will rise against him to kill him!
Did Nevu'ah come to a Na'ar (youth)?!
Radak: Yes! Targum Yonasan says so. Also Shmuel was a Na'ar when he received his first Nevu'ah. The spirit of Nevu'ah made them wise, even though they were youths. The Meforshim say that also Zecharyah ben Ido was young.
Radak: Perhaps "Na'ar" means servant. Even though he was old in years, he is called Na'ar, just like "u'Mesharso Yehoshua bin Nun Na'ar" (Shemos 33:11) Yirmeyah served a Navi or Chacham in his days, and said that it is proper to send someone greater than himself, like Moshe said "Shelach Na b'Yad Tishlach" (Shemos 4:13).