Olam Haba – From When He Answers Amen

The Gemara in Maseches Sanhedrin (110b) discusses the subject of a kattan, a child, who passes away- from which age does he merit Olam Haba, and the Gemara brings a few views. The Rem”a rules l’halachah like Rabi Meir, that a kattan merits Olam Haba from the time he answers amen, and wrote (Orach Chaim 124 7): "And he should teach his little children to answer amen, because as soon as a baby answers amen he has a share in Olam Haba."