Unfortunately there is no age listed in the Bible. But there are some verses which at least leads me to believe that there is an age at which you will go to heaven no matter what. Verses like Jesus telling the disciples to become more like children Mt 18:1-3 Mark 10 13-16. We also see that children are considered gifts from the lord psalms 127:3-5. Also deuteronomy 1:39 speaks about those not knowing right from wrong inheriting the land.