Is there a specific age at which children can truly be said to have capacity about

Is there a specific age at which children can truly be said to have capacity about life-altering medical decisions? Is it 18? Is it before or after? Explain your reasoning.

On the same note, to what extent should parents be allowed to make life-altering medical decisions for their children? Should the religious preferences of the parents impact the care of the child? Why or why not?