The truth about the 12th Imam
The Shia's 12th imam was not only 4 year old when he he became imam, but he was also "seen only by the elites of the shias" before he went into occultation.
He was born in Samarrah in 256/868 and until 260/872 when his father was martyred, lived under his father's care and tutelage. He was hidden from public view and only a few of the elite among the Shia were able to meet him.
After the martyrdom of his father he became the Imam(as) and by Divine.Command went intoGhaybat(Occultation) .
Thereafter he appeared only to his Na'ib (deputies) and even then only in exceptional circumstances.
Actually the 11th imam's brother was called "liar" by shias when he said that the 11th imam had really no son at all.
Ibn Babuya (d. 381/991) writes in Kamal ad-Din (1:222) that, "The 11th Imam's reported complaint that none of his forefathers had been as much doubted by the faithful as he was." Hasan al-Askari died in 260/874, but it however appears that none of the notables knew of the birth of al-Mahdi, and so they went to Jafar, the brother of Hasan al-Askari, assuming that he was now the Imam. Jafar was asked about the son of Hasan al-Askari, but he remained unshakable in his assertion that his brother had no progeny. For this reason, Jafar has been vilified in later sources as a liar (khadhhab).
Shia scholar Ayatullah Muntazari says:
The fact that his uncle Jafar was labelled a liar solely because of denying the existence of Mahdi has not been proven conclusively. Opposition of a few persons will not harm His being. During the course of history, many people have objected to various factual accounts solely due to their personal political or economical interests.
https://www.h-net.org/~bahai/trans/vol6/muntazir.htm
In this topic of shiachat, you will see shia discussing why the son of 10th Imam, the brother of 11th Imam, was a liar
Is Ja'far Al-kazab (i.e Liar) The Uncle Of Imam Mahdi(as)?
Nawbakhti , a shia historian, writes in his Fiaraq (p. 79) that, "Hasan al-Askari died and no offspring (khalaf) or vestige (athar) was seen after him. As no apparent child for him was known, his inheritance was divided between his brother Jafar and mother."
There is a huge controversy on whether this person even existed at all?