(a) Amr bin Salamah (may Allah be pleased with him) says that his father and a delegation of his people came to the Prophet (peace and blessings be upon him), when the people had accepted Islam, in order to learn the Qur'an. When they had finished their business, they asked the Prophet (peace and blessings be upon him): Who should lead us in prayer? He (peace and blessings be upon him) said: "The one who knows the most Qur'an." They came back to their people and asked, but no one had collected and memorised more of the Qur'an than I had. I was a boy at that time, wearing a sheet. They put me forward and I led them in prayer. He says: Whenever these people gathered, I would lead them in prayer. Mis'ar bin Habib says that he would also lead their funeral prayers and (lead them) in their mosques.
(b) Ayyub al-Sakhtiyani narrates from Amr (may Allah be pleased with him) that he was seven or six years old, and in one narration that he was seven or eight years old.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الجمعة / 5859