It is narrated from al-Hasan (may Allah have mercy on him) that the first to accept Islam was Ali bin Abi Talib (may Allah be pleased with him), and he was fifteen or sixteen years old.
Another narration has the words: The first to accept Islam after Khadijah (may Allah be pleased with her) was Ali (may Allah be pleased with him), and he was fifteen or sixteen years old at the time.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب اللقطة / 12164