Their book says that you should begin spanking as soon as a child shows a temper or disobeys...and that starts as young as 4 months of age.
Definately should be spanked by 6 months for wiggling during diaper changes, crying when put down, touching anything you've said 'no' about, crying when you say 'no' etc. One example in the book is spanking a 7mo for what they called 'pitching a fit' - crying himself to sleep while the mother rocked him.