The "old and wise ESB Yoda" was 900 years old in the OT. He was a lightsaber-wielding, twirling dirvish in ROTS set roughly 20 years before the OT . . . when he was a ripe young 880.
According to Yoda in ESB, he had been teaching Jedi students for 800 years! That was his role. When you combine that with his dialogue in response to Luke saying he was looking for a great warrior, and then his lesson that "A Jedi uses the Force for knowledge and defense, never for attack," I think it's a pretty clear picture that onscreen ESB Yoda matched Lucas' description in that 1981 conversation with Kasdan and Marquand.
In that conversation, Lucas says Yoda is "not a real Jedi." To me, it's pretty clear that he doesn't just mean that Yoda got too old for Jedi combat. He's a teacher; a guru. For 800 years! When people I know saw Yoda bust out a lightsaber in AOTC, the reaction was basically, "WTF!?"
Well it was written that way by his creator if you dont like yoda thats fine. You cant pick and choose aspects you like and say thats the only real yoda. Also in the book its explained he has to use up alot of force power to move like that at that age, which is why you see him with a cane when he isnt fighting in both atoc and sith. Moroever his race ages differently so its possible as they grow closer to death they weaken at an accelerated rate, since they age so slowly to begin with. Mando eleborates a bit more on this all i remember is baby yoda is actually 50 years old