Now his parents went to Yerushalayim every year for Pesach. When he was twelve years old, they went up to Yerushalayim for Yom Tov as usual. After Yom Tov, when they were on their way home, the child stayed in Yerushalayim without telling his parents, who figured that he was traveling with his cousins. After a day, when they realized that he wasn’t with them, they went back to Yerushalayim to find him. After three days, they found him sitting and learning from some of the greatest Gedolim, and could not only follow their shiurim, but could even upshlug them. Everyone who heard him were amazed at his understanding and at his answers.To see the full biography, see here.
[Hat Tip: ADDeRebbetzin]