Bayhaqi 12165 — Those who said: His Islam is judged valid

It is narrated from Sayyiduna Ibn Abbas (may Allah be pleased with them both) that the Messenger of Allah (peace and blessings be upon him) gave the banner to Sayyiduna Ali (may Allah be pleased with him) on the day of Badr, when he was twenty years old.
Imam Ahmad (may Allah have mercy on him) said: The Battle of Badr took place a year and a half after the Messenger of Allah (peace and blessings be upon him) came to Madinah. They differed over how long he stayed in Makkah after he was sent as a prophet: it is said ten years, it is said thirteen years, and it is said fifteen years. If it was ten, and it is correct that Sayyiduna Ali (may Allah be pleased with him) was twenty on the day of Badr, then his age when he accepted Islam comes close to what Urwah said. If it was thirteen or fifteen years, then his age was less than that.
They also differed over the age of Sayyiduna Ali (may Allah be pleased with him) at the time of his martyrdom: it is said sixty-five years, it is said sixty-three years, and less than that has also been said; the best known is sixty-three, at the end of forty years from the emigration of the Messenger of Allah (peace and blessings be upon him). So his age when he accepted Islam comes to thirteen years on the view of those who say the Messenger of Allah (peace and blessings be upon him) stayed in Makkah ten years, and to ten years on the view of those who say thirteen. Thus in most reports, when he (may Allah be pleased with him) prayed with the Prophet (peace and blessings be upon him), he had reached an age at which it is possible that he had attained puberty. The verses of poetry that are narrated are open to interpretation, besides their chain being weak. Moreover, the ruling that only the statement of one who has reached puberty is valid, and not that of a discerning child, was legislated after Sayyiduna Ali (may Allah be pleased with him) accepted Islam, so his Islam was judged valid: either because he lived on until he professed Islam after reaching puberty, or because the Prophet (peace and blessings be upon him) addressed him with the call to Islam while other boys were not addressed, or because at that time the statement of a discerning child was judged valid before the Shari'ah came with something else, or because he had already had a wet dream and so had reached puberty. And Allah knows best.
In the narration of Qatadah, al-Hasan al-Basri and others held that when Sayyiduna Ali (may Allah be pleased with him) accepted Islam he was fifteen or sixteen years old.