Ali (may Allah be pleased with him) was killed at the age of sixty-three

It is narrated from Shurahbil bin Sa'd al-Qurashi that Sayyiduna Ali bin Abi Talib (may Allah be pleased with him) was made caliph in the year 35 AH, when he was fifty-eight years and some months old. When the Hajj season of 35 AH came, he sent Sayyiduna Abdullah bin Abbas (may Allah be pleased with them both) as leader of the Hajj in the years 35, 36, 37, 38 and 39 AH. Then there came a Hajj season when Sayyiduna Ali (may Allah be pleased with him) was occupied with fighting, so the people agreed upon Shaybah bin Uthman al-Hajabi, who led the people in Hajj. Then when the year 40 AH came, Sayyiduna Ali (may Allah be pleased with him) was martyred on Friday, the 17th of the blessed month of Ramadan, when he was sixty-three years old.
Imam al-Hakim said: We reflected on these dates and found clear proof for them with an authentic chain.

It is narrated from Sayyiduna Abdullah bin Mas'ud (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) said: "The millstone of Islam will turn for thirty-five or thirty-six years. Then if they perish, it will be the way of those who perished; and if their religion endures for them, it will then endure for seventy years." Sayyiduna Umar (may Allah be pleased with him) said: O Messenger of Allah! Is this of the period that remains or of the period that has passed? He (peace and blessings be upon him) said: "Of the period that remains."
This hadith has an authentic chain (sahih al-isnad), but the two Shaykhs did not record it.
[Mention of the pledge of allegiance to the Commander of the Believers, Sayyiduna Ali bin Abi Talib (may Allah be pleased with him)]
Imam al-Hakim (may Allah have mercy on him) said: The narrations differ about the time of his pledge of allegiance. It is said that the pledge took place four days after the martyrdom of Sayyiduna Uthman (may Allah be pleased with him); some said five days and some said three days. One view is that the pledge took place on Friday, 25 Dhu al-Hijjah, and another view is that the pledge took place immediately after the martyrdom of Sayyiduna Uthman (may Allah be pleased with him) in the house of Amr bin Mihsan al-Ansari. But the most authentic narration is that he refused to accept the pledge until Sayyiduna Uthman (may Allah be pleased with him) had been buried; then he accepted the pledge openly on the pulpit of the Messenger of Allah (peace and blessings be upon him), and the first to pledge allegiance was Sayyiduna Talhah (may Allah be pleased with him), and he said: This is a pledge that will be broken.

It is narrated from al-Aswad bin Yazid al-Nakha'i that when the pledge of allegiance to Sayyiduna Ali bin Abi Talib (may Allah be pleased with him) took place on the pulpit of the Messenger, Khuzaymah bin Thabit (may Allah be pleased with him) stood before the pulpit and recited these verses: "Now that we have pledged allegiance to Ali, Abu al-Hasan (Ali, may Allah be pleased with him) suffices us against all the trials we used to fear. We have found him the best and most deserving of people to lead the people, for he is the greatest scholar and expert among Quraysh in the Book of Allah and the Sunnah of the Messenger. No one of Quraysh can even reach his dust when he sets out (into the field) riding his agile, lean-bodied horse. In him are all the good qualities that are found separately in other people, but all the beauty and excellence gathered in Ali is not found in anyone else."

It is narrated from Abu Rashid that when the news of the pledge of allegiance to Sayyiduna Ali (may Allah be pleased with him) reached Sayyiduna Hudhayfah (may Allah be pleased with him), he said: "After him I will not pledge allegiance to anyone who is lower than him in rank or who has no standing."
Imam al-Hakim said: All these reports that have come concerning the pledge of allegiance to the Commander of the Believers are authentic and agreed upon. As for the claim of those who assert that Abdullah bin Umar, Abu Mas'ud al-Ansari, Sa'd bin Abi Waqqas, Abu Musa al-Ash'ari, Muhammad bin Maslamah al-Ansari and Usamah bin Zayd (may Allah be pleased with them) held back from pledging allegiance to him, this is the statement of those who deny the reality of the circumstances of that time. Now listen to the reality of it: