Ephesians 6:1
Children, obey your parents in the Lord: for this is right.



This is an instruction directed at children, written in letters which are addressed "to the saints which are at Ephesus, and to the faithful in Christ Jesus:" Ephesians 1:1.  The only conclusion that can be drawn from this is that there are children in the body of Christ.  Children can become accountable to God before they are mature adults. 

See also Colossians 3:20 and then Colossian 1:1. 

Some of the saints and faithful at Ephesus and Colossi were children.