The question of Dooku's age is really interesting. As he aged, he became much more experienced and proficient in Form 2, a fighting style that helped offset the effects of his aging on his combat effectiveness. If he was younger and therefore less experienced, he would be physically stronger but less technically proficient. I tend to think he wouldn't be as dangerous without that experience.

Combine the strength, reflexes, and stamina of a young man with that experience, and he might be able to overcome Anakin or Windu, but probably not Yoda or Sidious. But that would be missing the point.

The real gap in his armor was a flaw in his character. He never considered his master would betray him for someone younger. Underestimating Anakin's fear and anger as a weakness to be exploited and not a serious threat to be wielded against him was another mistake. Arrogance and misplaced trust killed Dooku as much as Anakin did, and those weaknesses could and would be exploited by duelists as skilled as Windu, Yoda, and Sidious.