Great article on Training Frequency (Overtraining).
I think it is almost impossible for the non competitive bodybuilder to overtrain if they are paying appropriate attention to their diet, sleep patterns and stress levels. People give themselves too much credit on how hard they train. If you pay attention to your body and adjust training intensity accordingly I don’t think the majority of us have an issue. Especially with age and experience.
This article somewhat supports my views.
Source: Muscle and Fitness (Issue Aug 1980)