After the martyrdom of Ali ibn Musa al-Reza (peace be upon him) and the departure of Mamun from Khorasan to Baghdad, he wrote a letter to Imam Muhammad al-Jawad (peace be upon him) and, with great honor and respect for his knowledge, wisdom, and virtues, requested his presence.
This was a very difficult and burdensome matter for the Abbasid family because the Imam was both from the Banu Hashim family and in his childhood. Therefore, they opposed Mamun’s request. However, since Mamun was determined and recognized the Imam’s knowledge and excellence, he insisted on holding a debate between the scholars of the time and the Imam. Mamun arranged a grand assembly. After much questioning and answering, all of the opponents admitted to the Imam’s superiority despite his young age, and the Abbasids had no grounds left for objection.
In confirmation of this, the Imam himself, through a scholarly argument and by referencing the Quran, defended the idea that there is no contradiction between the young age of the divine proof (Hujjat Allah) and his God-given knowledge and wisdom:
Ali ibn Asbat says that at that time, when Imam al-Jawad was still a child, he was coming toward me. I looked at his appearance and form so that when I went back to my city, I could describe the Imam to my friends. While I was thinking about this, the Imam sat down and said:
“The proof of God in Imamate is like His proof in Prophethood. God says in the Quran, in verse 12 of Surah Maryam: ‘And We gave him (Yahya) wisdom as a child.’ And in verse 22 of Surah Yusuf, it says: ‘And when he reached his full strength, We gave him wisdom and knowledge.’ And in verse 15 of Surah Ahqaf, it says: ‘And when he reached his full strength and reached forty years of age.’
Therefore, according to the Quran, it is permissible for wisdom to be given to an Imam in childhood, just as it is permissible for wisdom to be given to him at the age of forty.”
Taken from the book History of the Fourteen Infallibles by Allama Muhammad Baqir Majlisi (with some editing), and the book Stories of Imam Kazim and Imam Jawad (peace be upon them) by Qasim Mir Khalafzadeh.