pukunui
Legend
The timeline was foreshortened in ESB when Lucas revealed that Vader was 22 year-old Luke’s father.I believe the timeline was at least somewhat foreshortened as of the prequel trilogy, so that they could round it out with the birth of Luke and Leia, but it's probably only a difference of 5-10 years
If you take ANH on its own, Obi-Wan’s age and all the comments about ancient religions and such do make it sound like a much longer time has passed since the Clone Wars and the end of the Jedi.
But by making Vader Luke’s father in the very next film, Lucas made that an impossibility … unless he wanted to have Vader father children after the fall. (I suppose in that sense it was the prequels that established that Luke and Leia are the same age as the Empire.)