I imagine it like this. 45 years prior to The Battle of Yavin, Obi-Wan was already like 35 years old and an active Jedi Knight, which is when Anakin was discovered and began his training. I also see the Clone Wars already having begun and continuing to about 40 years before Yavin. At this point, The Republic transformed into the Galactic Empire, due to inner corruption, bureaucratic crap, and due to the practical need for a stronger government, as there were obviously such great threats as the Clones and so on. In the beginning, The Empire was "a good idea", as Luke says of it in one of the deleted scenes from Star Wars.

I see The Empire having continued to support the service of the Jedi Knights in these early days, until Senator Palpatine crafts a grand conspiracy theory charging them with beginning the Clone Wars (which may be partly true as The Thrawn Trilogy has the cloned Jorus C'Baoth). The Purge doesn't begin although the Jedi have been exiled. I then see an early proto-Rebellion occurring from 35 to 20 BBY, and at this latter date Anakin becomes Darth Vader and Purges all of the Jedi save Yoda and Kenobi. Briefly before his fall at this date, Anakin would have had to have copulated with the mother of Luke and Leia.