It seems pretty obvious that Yoda is indeed getting old, and weak. His strength during the battle comes from the force. It flowed through him, giving him the one up on Dookuboy. If you notice after the fight, he seems drained.
I don't understand how people can consistently bitch about that. It's simple! Force powers = less pain, better movement, etc...! And as was said, he WAS the most powerful Jedi in the Republic.
Happiness is like peeing your pants: everyone can see it but only you can feel its warmth