How is it possible for some of the Imams to reach Imamate while still children?
Concise answer
According to the undisputed belief of Imami Shi'ism taken from numerous verses and Prophetic traditions, the Imamate is a Divine position. So if it is established that someone has been divinely appointed to this position a Muslim must submit to his command and authority without question obeying Allah (awj) as He says, "Allah best knows where to place His messengership."
There are several ways to ascertain whether Allah (awj) has appointed a particular person to this position:
1. Prophetic traditions.
2. Being introduced and specified by other Imams, particularly the previous Imam.
3. Possessing the other conditions of Imamate, such as inherent knowledge, more knowledge than anyone of their time, infallibility, soundness and moderation of body and spirit, and ability to perform miracles and extraordinary deeds.
The Shi'as contemporary to young Imams-that is, Imam Muhammad b. ‘Ali al-Taqi (ع) at eight years, Imam ‘Ali b Muhammad al-Naqi (ع) at nine years, and Imam al-Mahdi (ع) at five years-were not unaware of these things, and it was after great searching and effort that they attained certainty about their Imamate and submitted to their authority. Later generations in turn rely on research, historical records, and traditions. On the other hand, the position of Imamate, unseen Divine endorsement, and manifestation of extraordinary deeds at the hands of these personages prevent them from being compared to normal human beings.
In addition, for a Muslim who believes in the Qur`an, the granting of a Divine station on the basis of Divine wisdom, knowledge, and grace to someone of young age should not be surprising, since the prophethood of ‘Isa (ع), Yahya (ع), and Sulayman (ع) also took place at a young age. This is why the noble verses that indicate this point were frequently cited by the Imams (ع).
Detailed Answer
Imamate, in the view of Imami Shi'as, is the practice by the Wali (Divinely appointed authority) of his authority over people (wilayah); in other words, Imamate is a Divinely-appointed station for managing the people's worldly and religious affairs as well as guiding them to the peak of felicity and humanity. Thus, the Imam cannot be determined or selected by the people, since inherent knowledge and inner infallibility are unseen qualities which no one but Allah (awj) knows, and these are the two most important qualities of the Divinely-appointed Wali.
A monotheist who has submitted to Divine authority over him or herself is wholly obedient to the commands and prohibitions of his or her Allah (awj). Thus, in accepting the authority of the leaders appointed by Him as well, he or she must be fully obedient, submit to their authority, and avoid comparing them to others or baselessly second-guessing their appointment. There are several ways to recognize the divinely appointed Imam/Wali (guardian):
1. Studying their conduct and actions.
2. Referring to the attributes and signs that the previous Wali has mentioned for the succeeding Wali.
3. Seeking miracles and verifying the presence of the conditions and necessary corollaries of Imamate
It is possible to find out about their lives and character by referring to books of history and biography, and by looking at collections of traditions one can find the traditions that have come down from the Prophet (ص); and finally by examining at the traditions of each Imam, one can come to recognize the next Imam. The Imams (ع) performed miracles both in their lifetimes and after their martyrdoms. The occurrence of such extraordinary deeds for those who sought recourse to them is so frequent that it is beyond reckoning, and it is something that everyone searching for truth can experience.
So there is no specific age necessary to attain the station of Imamate. The spiritual, intellectual, and mental maturity required for the position can be granted by Allah (awj) from the day of his birth. This fact is one of the extraordinary phenomena that prove their Imamate, it does not detract from it. Obviously, from a superficial and shortsighted viewpoint, it appears difficult for prominent personalities, scholars and the old and young to submit to the authority of a boy; for ordinary people accepting such an authority would not be as easy as accepting older Wali.
The contemporaries to the young Imams-that is, Imam Muhammad b. ‘Ali al-Taqi (ع) who became Imam at the age of eight, Imam ‘Ali b. Muhammad al-Naqi (ع) who attained the Imamate at the age of nine, and Imam al-Mahdi (ع) who became Imam at the age of five-were no exceptions to this rule, and they posed this very question to Imam ‘Ali b. Musa al-Rida (ع).
For example, there are the following traditions: It has been narrated from Hasan ibn Jahm that he was in the presence of Imam ‘Ali b. Musa al-Rida (ع). Imam Muhammad b. ‘Ali al-Taqi (ع), still a child, was also present. After a long conversation Imam ‘Ali b. Musa al-Rida (ع) said to me, "Hasan! If I tell you this child is to be your Imam, what would you say?" I said, "May I be your ransom! Whatever you say, I say the same thing." The Imam (ع) said, "You speak the truth." Then Imam ‘Ali b. Musa al-Rida (ع) exposed the shoulder of Imam Muhammad b. ‘Ali al-Taqi (ع) and showed me a symbol that was similar to two fingers, and he said, "A comparable symbol was in the same place in the body of Imam Musa b. Ja'far al-Kazim (ع)."[1]
It has also been narrated from al-Mahmudi that he said, ‘I was in Tus (an area in Central Asia near present-day Mashhad) in the presence of Imam ‘Ali b. Musa al-Rida (ع). One of his companions said, "If something were to happen to the Imam, who would be the divinely chosen Imam after him? The Imam (ع) turned to him and said, "Regarding the issue of Imamate, after me refer to my son Jawad (Imam Muhammad b. ‘Ali al-Taqi (ع))." He (al-Mahmudi) said, "He is still a child!" Imam ‘Ali b. Musa al-Rida (ع) replied, "Allah chose ‘Isa (ع) as His messenger though he was younger than Imam Muhammad b. ‘Ali al-Taqi (ع)."[2]
In spite of these traditions some of Imam ‘Ali b. Musa al-Rida's (ع) followers out of anxiety still went in search of his successor. "Some of them went after ‘Abdullah ibn Musa, the brother of the eighth Imam (ع). Since they weren't willing to accept anyone's Imamate without evidence, they presented to him some questions, and when they found him unable to answer they forsook him." This is because what was important to them was the manifestation of this divine aspect in the Imams' knowledge. This is also why they would apply this principle to all the Imams and subject them to various questions, and only when they felt that they were able to answer them (along with the textual declarations of their Imamate) would the Imami Shi'as recognize them as infallible Imams. The young Imams were no exceptions to this rule; the leaders of the Shi'as would examine them to become certain of their level of knowledge and ability to perform clear miracles.
On the other hand, the enemies of the Imams, who were always waiting for an opportunity to marginalize the Imams and disperse the Shi'as from around them, did not sit by idly. Arranging intellectual gatherings and the like they strove to make the young age of these Imams an excuse to altogether marginalize them from society. But the more effort they made, the more they debased themselves, and the superior knowledge of the Imams stood out to their knowledgeable contemporaries.
In addition, it is obvious to those familiar with the Qur`an and stories of the prophets that several of them reached the station of prophethood, messengership and even Imamate at a young age, like ‘Isa (ع)[3], Yahya (ع)[4] etc. The majority of prophets reached this stage, however, at the age of forty or older.
In sum this issue should be referred to the Divine wisdom and knowledge, and as in all judgments one should make use of all available contexts, evidences, and proofs. As Allah (awj) says
"Say, O Lord! Yours is the dominion of the sky and the earth; You give the kingdom to whom You please and take it from whom You please, and You grant honor to whom You please and debase whom You please. All good is in Your hands. Verily You have power over all things."[5]
Allah (awj) explains similar objections of the Tribes of Israel and replies in these words:
"The Prophet told them, Verily Allah has made Talut your king. They said how can he be king, while we are more worthy of kingship than he, and he is lacking in wealth? He [their prophet] said Allah has chosen him over you and has granted him intellectual and physical abilities, and Allah gives His kingdom to whomever He pleases, and Allah is all-encompassing, all-knowing. And the prophet said to them, Verily the sign of His kingdom is that there shall come to you the chest in which there is tranquility from your Lord and residue of the relics of what the children of Musa and the children of Harun have left, the angels bearing it"[6]
[1] Mowsu‘ah al-Imam al-Jawad, vol. 1, pg. 137:
وَرُوِیَ عَنْ الْحَسَنِ بْنِ الْجَهْمِ قَالَ: دَخَلْتُ عَلى الرِّضَا ( ع) وَأَبُو جَعْفَرٍ ( ع) صَغِیْرٌ بَیْنَ یَدَیْهِ فَقَالَ لِیْ بَعْدَ کَلاَمٍ طَوِیْلٍ جَرى لَوْ قُلْتُ لَکَ یَا حُسَیْنُ إنَّ هَذَا إِمَامٌ مَا کُنْتَ تَقُوْلُ؟ قَالَ: قُلْتُ: مَا تَقُوْلُ لَهُ لِیْ جُعِلْتُ فِدَاکَ. قَالَ: أَصَبْتَ، ثُمَّ کَشَفَ عَنْ کَتِفِ أَبِی جَعْفَرٍ ( ع) فَأَرَانِیْ مِثْلَ رَمْزِ إِصْبَعَیْنِ فَقَالَ لِیْ: مِثْلُ هَذَا کَانَ فِی مِثْلِ هَذَا الْمَوْضِعِ مِنْ أَبِی مُوْسى صَلَوَاتُ اللهِ عَلَیْهِ.
[2] Dala'il al-Imamah, pg. 388
عَنِ الْمَحْمُودِی قَالَ: کُنْتُ وَاقِفاً عَلى رَأْسِ الرِّضَا ( ع) بِطُوسِ فَقَالَ لَهُ بَعْضُ أَصْحَابِهِ: إِنْ حَدَثَ حَدَثٌ فَإِلـى مَنْ؟ قَالَ: إِلـى ابْنِی أَبِی جَعْفَرٍ. قَالَ: فَإِنِ اسْتَصْغَرَ سِنُّهُ؟ فَقَالَ لَهُ أَبُو الْحَسَنِ: إِنَّ اللٌّهَ بَعَثَ عِیسى بْنَ مَرْیَمَ قَائِماً بِشَرِیعَتِهِ فِی دُوْنِ السِّنِّ الَّتِی یَقُوْمُ فِیْهَا أَبُو جَعْفَرٍ عَلى شَرِیْعَتِهِ.
[3] Surat Maryam (19), Verses 29-30:
} فَأَشَارَتْ إِلَیْهِ قَالُوا کَیْفَ نُکَلِّمُ مَنْ کَانَ فِی الْمَهْدِ صَبِیًّا. قَالَ إِنِّی عَبْدُ اللٌّهِ آتَانِیَ الْکِتَابَ وَجَعَلَنِی نَبِیًّا... {
[4] Ibid., Verse 12:
} یَا یَحْیَى خُذِ الْکِتَابَ بِقُوَّةٍ وَآتَیْنَاهُ الْحُکْمَ صَبِیًّا {
[5] Surat Ali-'Imran (3), Verse 26:
} قُلِ اللٌّهُمَّ مَالِکَ الْمُلْکِ تُؤْتِی الْمُلْکَ مَنْ تَشَآءُ وَتَنْزِعُ الْمُلْکَ مِمَّنْ تَشَآءُ وَتُعِزُّ مَنْ تَشَآءُ وَتُذِلُّ مَنْ تَشَآءُ بِیَدِکَ الْخَیْرُ إِنَّکَ عَلَى کُلِّ شَیْءٍ قَدِیرٌ {
[6] Surat al-Baqarah (2), Verse 247:
} وَقَالَ لَهُمْ نَبِیُّهُمْ إِنَّ اللٌّهَ قَدْ بَعَثَ لَکُمْ طَالُوتَ مَلِکًا قَالُوا أَنَّى یَکُونُ لَهُ الْمُلْکُ عَلَیْنَا وَنَحْنُ أَحَقُّ بِالْمُلْکِ مِنْهُ وَلَمْ یُؤْتَ سَعَةً مِنَ الْمَالِ قَالَ إِنَّ اللٌّهَ اصْطَفَاهُ عَلَیْکُمْ وَزَادَهُ بَسْطَةً فِی الْعِلْمِ وَالْجِسْمِ وَاللٌّهُ یُؤْتِی مُلْکَهُ مَنْ یَشَآءُ وَاللٌّهُ وَاسِعٌ عَلِیمٌ {
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