I don’t think any start date for the Empire was ever really implied by OT dialogue. But we can piece a few things together to work it out:
We know Vader must have turned evil no later than 19 years before ANH (due to Luke’s age), and we know Vader hunted down the Jedi Knights - meaning the Jedi Knights existed 19 years before ANH. We know the Jedi Knights were guardians of peace in the “Old Republic”. We can take this to mean the Old Republic existed 19 years before ANH. All of these add up to sort of give the picture that the Old Republic fell around the same time Vader turned evil and hunted the Jedi Knights.
So:
- Vader turned evil no later than 19 years before ANH
- Vader killed the Jedi Knights
- The Jedi Knights were guardians of peace in the Old Republic
Therefore… the Old Republic must have become the Empire 19 years before ANH. This is also what the Prequels show. So case closed.
Or IS IT??!??!???
Yes. It is. Mostly. But there’s just one nagging thing. Did anyone who grew up watching the OT actually feel like the Empire was only 19 years old?
I certainly didn’t. I had assumed the Empire might have been like centuries old. Most of the characters didn’t seem to even remember a time before the Empire, except for Obi-Wan, who, as a Jedi, may be supernaturally old. And the way Alec Guinness waxed poetic about the Old Republic - before the dark times, sounds like he was reminiscing for an ancient Golden Age of Legend. Something that existed in the remote past. And the way Han scoffed at Kenobi’s superstitious beliefs, or the way Tarkin spoke of Obi Wan as if he should be long dead, or Vader’s ancient religion.
The Empire never seemed only 19 years old. It seemed ancient - a Galactic Order that had existed as long as anyone can remember. That’s what made toppling it seem all the more spectacular.
I originally blamed the Prequels for showing the Empire lasted less than 20 years. But actually, the OT more or less implies this as well if you put the pieces together.