[Frapster]

If corporal punishment is involved at the age of 13 - the parent failed the child at least 4 years earlier.

[TalonDJ]

Even 13 yearolds might rarely need it. But yeah, by then it should be more or less no existent. At around that age the butt becomes more resistant to pain, and the judgment of the kid is developed enough that other forms of punishment have more weight.