Personally I think there is never an age when hitting a child is ok. With so many other valid options for discipline why hit?
At 15 and older there are even more options for discipline. In many places hitting a child who is almost an adult is considered assault. Also what 15 year old is going to sit still and willingly take a spanking? So then what? Do they get forcefully held down?
Teenagers are going to be disrespectful sooner or later. It is normal development that can be corrected with good parenting. However if a teen never got treated with respect in concern to discipline while growing up then the behavior has been taught.
Children are not possessions. They are people who deserve respect even when they are not perfect. Parents should find ways to discipline children which doesn't include hitting or screaming.