One of the lines of evidence pointing to Obi-wan being older than Guinness’s age would be the apparent age of Anakin at his death. He was played by Sebastian Shaw, who was about 78 at the time of filming. Assuming that Anakin is intended to be this old, If ROTJ took place around five years after ANH, that would mean that the birth of the twins occurred when Anakin was 54. This quite old, but not impossible by any means.
Also, if the Force allows people to live beyond their years, Anakin’s apparent old age in ROTJ could undercount his age as much as it could overcount it due to theories that the Dark Side may accelerate the aging process. As such, I tend to think that the actor’s age in ROTJ is a fair estimate.
If we conclude from this evidence that Anakin was middle-aged when the twins were born, this implies that either he killed the Jedi at this time, or that he killed the Jedi when he was younger and the twins were born years after he turned evil: “A young Jedi named Darth Vader…” Of course, ‘young Jedi’ to Obi-wan may mean anyone not pushing eighty. Recall that Yoda lived for 900 years.
With characters such as Han Solo not believing in the Jedi, it makes sense that the Jedi were mostly destroyed before Han was born, which means that Anakin could have begun killing the Jedi a decade before the birth of the twins, which would place the Empire’s start at thirty years before the battle of Yavin.
If this is accurate, then Obi-wan’s statement that Anakin was a young Jedi at the birth of the Empire makes a bit more sense, since he would have been about 44. This is still not ‘young’ as most people would define it, but even the current canon implies that most Jedi are old before they gain mastery of the Force.
The concept that the Empire arose over a ten year period of increasing strife and instability as the Jedi were hunted down and destroyed tracks better with Obi-wan’s statement about the ‘dark times’, and Luke’s astonishment that Obi-wan could have fought in the Clone Wars, since they presumably ended over ten years before Luke was born.