I think I stopped spanking my boys when they were 10 and 12. I just remember they did sometbhikng disobedient in the kitchen, and they started laughing. Then they asked me if they should bring me another wooden spoon to break over their rear ends. Then the older one, still laughing hysterically, pulled his pants down, and said, "Go for it mom!" I tried to spank him, but he had gotten me laughing. Then, I looked at their butts, and saw these rock hard gluts, and finally figured out why I was breaking spoons over them.They played hockey year round, and they were tough as nails.
After we all finished laughing I never spanked them again. But, they really were well behaved. The spanking on the lower end really establishes itself by the time they are 10.
PS. My daughter, the social worker, tells me spanking is against the law in Canada. You need not wonder why this next generation is so spoiled and entitled.