When Jeremiah attempts to recuse himself from his calling to be a prophet he does so by saying that he is unqualified because he is a child:
6“Ah, Lord GOD,” I said, “I surely do not know how to speak, for I am only a child!” 7But the LORD told me: “Do not say, ‘I am only a child.’ For to everyone I send you, you must go, and all that I command you, you must speak
Do Jewish scholars understand this to be humble self-deprecation on his part or was he actually prepubescent?
Rashi points out that the word "Alas" at the beginning of the sentence suggests that the prospect of serving as a prophet was distressing for him to contemplate. He also seems to suggest that by "I am a child" he was saying that he was "unproven":
for I am a youth: I am not worthy to reprove them. Moses reproved them shortly before his death, when he was already esteemed in their eyes through the many miracles that he had performed for them. He had taken them out of Egypt, split the Reed Sea for them, brought down the manna, caused the quails to fly, given them the Torah, brought up the well. I come to reprove them at the beginning of my mission.
I always read this as self-deprecation but now it seems more reasonable to take it at face value, that he was still wet behind the ears.
Literally a child or figuratively?
I have more pondering of this in this related question on another site:
Modern Judaism has the concept of a Bar Mitzwah that establishes an age of adulthood but there doesn't appear to be conclusive evidence as to when this official age of maturity was formalized. If it were that ancient then we would think that Jeremiah was not yet 14 years of age and/or physical adulthood.
So is there any reason to think that Jeremiah would have been pointing out that he had not yet reached puberty and therefore Jewish law or custom would exclude him from his calling?