At what age should an infant or young child be prevented from passing between a worshiper and his Sutrah, and is the Sutrah considered violated if the child passes despite an attempt to prevent him?

A Muslim must use a sutrah when praying, whether as an imam or alone, and prevent anything from passing in front of him. A young boy is also to be prevented from passing, due to the generality of the proofs. If the worshipper strives to prevent the passerby but is unable to do so, his prayer is valid, and the sin is upon the passerby.