Just taking the movie aspect out of it, events of ROTJ takes place 23 years after the fight on Mustafar. Vader has been a practicing Sith all that time. Prior to that he was training as a Jedi from the age of 8 or 10.
The events of ROTJ takes place 1 year after Empire (although the novelization gives the time period as six months), so that is how long Luke has been studying, on his own, since leaving Yoda to save Han.
So, cut the kid some slack eh?