I think it's much simpler. In Ep IV, Vader is pretty mechanical and Obi-Wan is pretty old. Quite simply, they just don't move like they used to. Luke hasn't been training his whole life like he would in the good ol' days of the Jedi Order, so he's actually more on par with these geezers. He does advance tremendously between IV and V, doing his own somersaults and backflips in the latter.

Still, the whole notion that you had to be trained from age 0 was asinine. I'm just good at rationalizing the minutae for the better enjoyment of the saga.

edit: I'm actually watching ESB right now thru Pinnacle. Pinnacle's preview option reverses the reds and the blues (why, I don't know). But I'm at the lightsaber duel and it's freaking me out. Vader's saber is blue, Lukes is red. Plus, all of the bluish lighting is now red, so it looks like a Tarantino flick. Totally bizarre.