Imam Javad (A.S.) was young, at the age of seven, when his father passed away. Books of history and biographies record some of these discussions about the personality of the young Imam (A.S.) and his capability for the leadership at that age.
When Imam Muhammad al-Javad (A.S.) was born, the same questions were sent to Imam al-Reza in order for them to know who would be the Imam after al-Reza (A.S.).
Was not Muhammad al-Javad (A.S.) a child?
And how could he bear the responsibility of the leadership and its affairs if he was at this age?
Indeed, Imam al-Reza (A.S.) always confirmed that the Imam after him would be his son, Muhammad Taqi al-Javad (A.S.), and the one who was most qualified.
Ibn Qulawaih, on the authority of Kulaini, on the authority of Hussein bin Muhammad, on the authority of Khariani, on the authority of his father, said:
"I (i.e. Khayrani’s father) was standing in front of Abul-Hassan al-Reza (A.S.), in Khurasan. Someone asked him: "My master, if something happens, to whom authority will belong?"
"To Abu Jafar (al-Jawad), my son," he replied.
The speaker indicated that the age of Abu Jafar was too young. So, Abul-Hassan al-Reza (A.S.), replied: "Allah, may He be praised, sent Jesus, son of Mary, to be an apostle, a prophet, the bringer of a revealed law (Sharia), to begin (his mission) when his age was younger than that of Abu Jafar(al-Javad), peace be upon him."
Narrated Muammar bin Khallad that he heared Imam al-Reza (A.S.), saying: "When he mentioned something and, then, said: "What need have you for that? Here is Abu Jafar, whom I have brought into my meetings and whom I have made to be my successor." Then, he added: "We are the family of the House (Ahlul-Bayt). Our young inherit from our old, like one feather (on a wing) followed by the next
Allamah Majlisi narrated, in his book "Uyoon Mu"jizat", a description of certain events of that critical period from the age of leadership, and which included the following text:
"... it was pilgrimage season. Some jurisprudents (Fuqaha") of Baghdad and other nations and their religious scholars, numbering 80, went to Medina to perform Hajj ceremonies and, then, they intended to see Abu Jafar Imam Javad (A.S.). When they reached the house of Jafar al-Sadiq (A.S.), because of its emptiness, entered it and sat on a big carpet.
"Then, Abdulla bin Musa (Imam al-Reza’s brother) came and sat at the head of the meeting. A caller stood and said: "This is the son of the Messenger of Allah (s.a.w.), therefore, whoever has any question, and he may ask him." He (Abdulla bin Musa i.e. Imam al-Javad’s nephew) was questioned about things, to which he inappropriately answered. His answers caused the Shi"a to be bewildered and grieved, therefore, the jurisprudents were confused among themselves and started leaving the pleace and telling each other that if Abu Jafar (the Imam) was present, he would have been able to answer all the questions addressed to Abdulla bin Musa.
"Then, a door was opened at the front of the meeting and Muwafaq, the Imam’s servant, entered and said: "This is Abu Jafar Imam Javad (A.S.)." All those who were present stood up, welcomed and greeted him, and then the Imam (A.S.) entered.
He (A.S.) sat while all people kept silent. Then, the questioner stood up and asked the Imam (A.S.) different questions, to which he (A.S.) answered perfectly and correctly. His answers caused those present to be happy, give thanks and praise the Imam.
They told him: "Indeed, your uncle, Abdulla, gave verdicts so and so." He (A.S.) said: "There is no god but Allah, o my uncle, Abdulla, gave verdicts so and so." He (A.S.) said: "There is no god but Allah, O my uncle! It is great to stand tomorrow (on the resurrection day) before His hands and He will say to you: "Why did you give verdicts (issue religious decrees) to My servants about things you did not know, while there was someone among people who had more knowledge than you."
Even religious scholars and jurisprudents confirmed their declaration for the Imam Javad’s being qualified with the position of leadership. The vivid declaration of Imam al-Reza (A.S.), for his leadership, and the acknowledgement of the famous traditionist, Ali bin Jafar, are, also, proofs for the Imam’s qualification for the position of leadership.
He became an Imam at the age of nine. Mamun thought that as all the rulers before him had oppressed the Holy Imams and their schemes had backfired that he would try to bribe the Holy Imams. He tried to make the 8th Imam his heir apparent and give him power and wealth but that also backfired.
He now tried to use power and wealth with the 9th Imam again but from a much younger age thinking that he would be able to influence him.
His main purpose was also to make sure that the 12th Imam (whom he knew would bring justice to the world) would be from his progeny and therefore intended to give his daughter Ummul Fadl to the Imam for a wife. Mamun still continued oppressing the family and followers of the Ahlul-Bayt (A.S.).
Mamun called the young Imam (A.S.) to Baghdad from Madina and offered his daughter. This infuriated his family (Banu Abbas). To prove to them the excellence of Imam even at a young age he arranged a meeting between Imam and the most learned of men at that time - Yahya bin Athkam, the Chief Judje of his period .
Imam (A.S.) read his own Nika (the khutba of which is used today) with the Mehr of 500 dirhams. Imam wrote a letter to Mamun that he would also give Ummul Fadhl Mehr from the wealth of Aakhira.