The OT doesn't concretely nail down the date of the Empire's formation.

One could infer from Ben's dialogue in the first film that it happened relatively late in the lives of Ben and Anakin, but that's only if you decide to take his words at 100% face value. Personally, I choose to be more liberal with my approach and run with the idea that the Republic became the Empire over fifty years before the events of the original movie -- before the Clone Wars came to an end, even before Palpatine rose to power -- and interpret Ben's comments about the Old Republic as reflecting a longing for a past golden age he's heard much about but never personally experienced.