There is definitely no age requirement for salvation. Jesus Himself declared, “Let the little children come to me, and do not hinder them, for the kingdom of heaven belongs to such as these” (Matthew 19:14). As soon as children are old enough to understand that they have sinned (Romans 3:23), that Jesus died to pay the penalty for their sins (Romans 5:8; 6:23), and that they must place their faith in Jesus for salvation (John 3:16), then they are old enough to be saved.
A child does not have to understand all the complex issues that are part of the doctrine of salvation. The Bible often encourages us to have faith like a child (Matthew 18:4; Mark 10:15; Luke 18:17). It is important that parents make sure that their children understand the basic issues (as described above), but the promise of Acts 16:31 is equally true with regard to an adult or a child: "Believe in the Lord Jesus, and you will be saved."
Little children, whether born of believers or unbelievers, may be chosen of God, redeemed by the blood of Christ, and have the work of the Holy Spirit in their hearts, and so enter into heaven. At what point in their lives they come to a realization of these things will vary from child to child. Some young children have especially tender hearts and, upon hearing that Jesus died for them, are immediately aware of their sinful natures and are compelled to respond. Others of more sanguine personalities may not come to this awareness until they are much older. Only the Lord knows the thoughts of the heart and we trust Him “to seek and save that which is lost” (Luke 19:10) according to His perfect will and timing.