“The first trilogy is about the young Ben Kenobi and the early life of Luke’s father when Luke was a little boy, some twenty years before the second trilogy which includes #StarWars and Empire... The entire saga spans about fifty-five years.” - George Lucas, October 1979 
At the time of this quote, George was probably thinking that Luke was about 25-years-old (the same age as Mark Hamill) during the OT, allowing Luke to be a young boy during the PT. And Luke and Leia weren’t twins yet, a change which lowered Luke’s age.
For more on George Lucas’ ever-changing plans for a prequel and sequel (and possibly fourth) trilogy of #StarWars films, I highly recommend reading @jwrinzler’s October 2012 article on the subject.
starwars.com/news/the-long-…
starwars.com/news/the-long-…
