Children cannot be saved until they’re old enough to understand the gospel clearly and embrace it with genuine faith. But you start as soon as you can teaching them and God knows when that heart readiness has come. People always ask me, “At what age?” It’s different for every child and different in every circumstance. But they do need to be mature enough to understand sin and righteousness, to understand repentance and faith, to understand punishment. They need to be old enough to understand the seriousness of their sin, the nature of God’s holy standard. At what age? It varies from child to child. But at the beginning you just start teaching and teaching and teaching and as they develop that understanding, God will work His work when they reach the point of comprehension.
John MacArthur