Imam is the Representative of Allah

After the martyrdom of Imam Ali ibn Musa al-Reza (A.S.), Ma’moon moved his central power in Khurasan to Baghdad. He then sent a letter to Imam Muhammad Taqi al-Jawad (A.S.) and summoned him respectfully and honorably acknowledging his knowledge and superiority.
The elders of the Abbasid Dynasty immensely disagreed with this matter because the Imam was not only from the Hashemi tribe, but he was also a child. Ma’moon however convinced the Abbasids through arranging a debate between the contemporary scholars and Imam Jawad (A.S.) He set up an enormous debate session. After the Abbasids saw the Imam defeat all other scholars, they had no other choice, but to agree with Ma’moon suggestion.
Imam Jawad (A.S.) has explained the possibility of being a child and yet a representative of Allah (SWT) who defends the religion of Allah (SWT) through the Holy Quran, and logical and scientific reasoning.
Ali ibn Asbat narrates: One day when the child Imam Jawad (A.S.) was coming towards me, I deeply stared at him to memorize his appearance. I wanted to describe him for my friends when I returned to my town.
Imam Jawad (A.S.) interrupted my thoughts as he sat and said: “Allah’s representative (Hujjat) in Imamat is similar to His representative in Nubuwwat. Allah in Quran says: ‘And We gave him the command for prophethood in childhood’ (the Holy Quran, 19:12. He also says: ‘When Yusuf attained his full manhood, We gave him power and knowledge’ (the Holy Quran, 12:22). Furthermore He says: ‘And when he reached the age of full strength and attained forty years’ (the Holy Quran, 46:15).
So, he deserves to be offered wisdom in childhood, as he is in the age of forty.”
Source: The History of 14 Infallibles, by Sayyid Muhammad Baqir Majlisi