We know the 13 yo thing, but really; how can we tell for sure?
Matthew 19:14 "But Jesus said, Suffer little children, and forbid them not, to come unto me: for of such is the kingdom of heaven."
One of the few times I prefer the KJV to NIV. Jesus was pretty clear about sin and pointing it out to those around Him. The comparison to adults with children is stark. Jesus loved having children around Him and they loved being with Him. His direction that we should be like children in accepting the Kingdom of God is quite revealing and one that makes such a beautiful picture of His Holiness.