Nigel writes: Yes, but where to draw the line? Some adults have the 'mental age' of children, and some children are exceptionally precocious.

A child is a child. It is inappropriate to sentence a child as an adult.

By "trying as an adult" the suggestion is already that the person is guilty, which is against law and custom in all but the most retrograde legal systems.

Someone with the mental age of a child should be treated appropriately; there should be sentencing guidelines for this purpose.