It’s really simple: still 19 years just like in the prequels.
Vader is said to be instrumental in founding the Empire and killing the Jedi. Luke is 19. Whether Anakin and Vader are the same person or not, Anakin would need to be alive and on the light side 19 years ago so he can provide his part in Luke’s birth; BEFORE he’s “killed”/turns to the dark side. It’s kinda just simple math.
Unless Vader conceived of Luke while he was a Sith lord, but there’s OT dialogue that contradicts that (“…if Anakin had any offspring…”). Plus the sequence of events wouldn’t make any sense; I don’t see Luke ending up under the supervision of Obi-Wan if he was conceived while Vader is evil, he’d absolutely end up with the Empire.
It’s been a few years since I last commented on this thread, but if I remember correctly, there was some debate about how the formation of the Empire worked out pre-PT. There was some information to suggest that the Empire could have been formed long before Luke and Leia were born and that the Jedi fought against the Empire/Sith as an opposing faction until they were eventually wiped out. This would then make Anakin’s fall to the dark-side happen after the formation of the Empire, and not concurrently as it does in canon.
This is also how things more-or-less were in the 1st draft of SW, were there were several, though not many, Jedi still around fighting against the Empire and multiple Sith Lords. So it’s possible that this was still in Lucas’ mind during the making of the OT, though by the time of ROTJ this had probably changed to something a bit more like the final canon established in the PT.