I think once you've been training hard and know the basics of eating for 3-4 years, you have built most your muscle mass naturally that you ever will.
When you are older, over 35, think it is best to focus on MAINTAINING and better condition. Of course,drugs change the whole equation.
On the Heavy Duty type training, I think it is to stressful as you age CNS, joints, etc. Also, you can't continue to increase poundages/size ad infinitum, or everyone would have a 800lb bench press and 20" arms.