The whole age thing is probably the most glaring and significant continuity error in the prequels. I'd say its the biggest mistake Lucas made, period. Episode I Anakin should have been the age of Episode II Anakin, and the same goes for Obi Wan. They are both 10 years too young by the time RotS ends.
The other two biggest mistakes I think Lucas made were odd and misplaced dialog that was either uneeded or out of character, and his need to put all sorts of little special effects into important scenes that distract from the main action. Do we really need stuff like the lava droid flying by the action during the A/OW duel at the end of III?
I for one consider Episode III to be all I need of the prequels at this point (it truly does have some great, classic Star Wars moments in it). I consider II to be a complete waste, but I do think Episode I has some good stuff in it. I think it just needs a good, strong editor to tighten it up.
I dunno, the "Seven Samurai" template always seemed like a natural one for Lucas to follow for the prequels, and he didn't. I never understood that...