There’s kind of a “thematic dissonance” in the OT about this. In ANH, you really get the sense the Empire is a very old institution - the way Kenobi talks about the Old Republic, before the dark times, or how he tells Luke he hasn’t gone by the name Obi Wan for “a long time… a long time.” Plus, the whole epic fantasy vibe and the implied backstory of a Golden Age Old Republic that lasted for “a thousand generations” sort of suggested longer time scales for all of this.

But then in the Prequels we find out the Empire is less than 20 years old. But a closer examination reveals the Prequels aren’t to blame here: it’s actually the fault of Empire Strikes Back. Turning Anakin into Vader had the unintentional side effect of heavily implying the Empire is about the same age as Luke, since Luke’s father is now Darth Vader, who killed the Jedi Knights of old. (Separate Anakin/Vader characters allows the possibility that Luke’s father was among the last of the Jedi Knights, who were victims of an ongoing multi-decade purge carried out by Vader under the Imperial banner.)

The pre-Prequel Gen-X fans also picked up on this. Early fan-fiction stories about the Prequel era, written in the 1980s, generally set the Prequels around 20 years before ANH, and considered the birth of Luke/Leia to be cotemporaneous with the rise of the Empire and the fall of Vader.