Al-Najib bin al-Sari (may Allah have mercy on him) said: Sayyiduna Ali (may Allah be pleased with him) said: I preceded them to Islam while I was a boy who had not yet reached puberty.
This saying is widespread among people as a saying of Ali (may Allah be pleased with him), but it has not reached us with a chain of the kind that can be used as evidence.
The people of knowledge differed over the age of Sayyiduna Ali (may Allah be pleased with him) when he accepted Islam.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب اللقطة / 12159