ki na'ar anochi -- is there an age requirement to becoming a navi?

In the haftarah to Parshas Pinchas, Yirmiyahu says to Hashem that he is unfit to be a prophet because he is a “na’ar.” Most of the translations I’ve seen render this as “a lad” or “young boy“ or something similar. There are two problems with this translation:

Binyamin is called a na’ar by his brothers, but when he appeared before Yosef he was already married and had 10 children. Yosef is called a na’ar (“v’hu na’ar es bnei Bilha v’es bnei Zilpah…”) even though at that time he was a teenager. Yehoshua (“u’meshorso Yehoshua bin Nun na’ar lo yamish m’toch ha’ohel”) must have been a middle aged man and he is still called a na’ar.

Even granting the possibility that na’ar here means young man, why should Yirmiyahu’s age have been a barrier to nevuah? Radak points out that Shmuel was able to receive nevuah when he was very young -- why not Yirmiyahu?

Radak explains that the word na’ar means an apprentice or someone in training. Yehoshua was Moshe’s apprentice; Yosef may have been serving his older brothers, bnei Bilha and Zilpah. Yirmiyahu’s argument was not simply that he was too young to be a navi. What he was saying is that he was still merely an apprentice navi – there was someone above him, greater than him, who G-d should have chosen. This echoes Moshe’s argument that since Aharon was an experienced, established navi, there is no reason to select him to deliver G-d’s message.

I wanted to just add that this may not just be a practical argument, but it may be a halachic argument. The gemara (Meg 14b) asks how Chuldah could prophesize while Yirmiyahu, who was a greater navi, was around. (I’m not 100% sure I understand the gemara’s question – if G-d starts speaking to you, how can you not prophesize, regardless of how much greater others may be? I assume what the gemara means is that receiving nevuah requires some kind of prior preparation. If there is someone greater already acting as a navi, that bars others from making those necessary preparationsto receive nevuah themselves.) The gemara answers that since she was a relative of Yirmiyahu’s, he was mochel. It sounds like there is a din similar to that of moreh halacha b’makom rabo with respect to nevuah. Maybe this is what was bothering Yirmiyahu – as a talmid, or apprecntice navi, he was in a similar position as Chulda and al pi din had to defer to the greater navi.