"Before the Dark Times... before the Empire." -Obi-Wan

"The last remnants of the old republic have been swept away" -Tarkin

These statements are ambiguous enough that the timeline is fairly open.

Obi-Wan was apparently alive to be a Jedi during the good old days. If he was 70, that puts the Empire at maybe 40 years old.

Possible Chronology for an "Old Empire" theory.

70 BSW Obi-Wan is born
50 BSW Obi-Wan serves as a Jedi Knight, has adventures in The Clone War I
40 BSW After the devestation of CW1, the Galactic Empire is founded, a beacon of hope.
35 BSW More Clone Wars, and the Empire tightens its grip. Obi meets Anakin during a mission.They serve the still idealistic, but darkening Empire
20 BSW The Jedi are all but eliminated as the Purge happens and the Empire keeps gettign worse.
1 BSW After losing power for years, the senate is offically disolved

In this possible scenario, Obi-Wan is personally betrayed by the Empire as opposed to fighting it from the beginning. Dramatically it's worth contomplating.

A New Empire, with a strong government to keep the peace and promote justice. It just goes bad within a decade or so as the Emperor shows his true colors... Palpatine is a Napoleon type.