John Doom said:

About Obi-wan's age, I found this dialogue useful:

                                     VADER
                         A tremor in the Force. The last time
                         I felt it was in the presence of my
                         old master.

                                     TARKIN
                         Surely he must be dead by now.

                                     VADER
                         Don't underestimate the power of the
                         Force.

Tarkin seems to imply that Obi-wan may be very old, so much that he may've died of old age, yet he survived with the power of the Force. Could he be more than 70 years old? That would make sense: Yoda was able to live for more than 800 years in the same way.

Vader, on the other hand, seems he's not considered "old" by Tarkin, thus it's possible he's still in his 40s-60s in Star Wars.

If that's the case, Anakin's could've actually been the last Jedi generation, explaining why few people know about the Jedi in the OT.

Finally, if the Jedi disappeared with the rise of the Empire, it would make sense it started no more than around 50 years before Star Wars.

The shooting script for ANH specifies that Obi-Wan is at least 70 years old. I believe this remained the general school of thought until the prequels came along and mucked about with the original timeline.

(Said script also specifies that Luke is 20 and Leia is 16 -- meaning they're definitely not twins.)