There have been several post-canonical Oz books and stories that deal with Lurline's enchantment, most of them not really agreeing on significant points. As someone who likes to tie together as many sources as I can, I think it might be prudent to take a look at some of them.
Marcus Mebes' Lurline and the White Ravens of Oz is one of the most detailed descriptions of the enchantment, but also leaves a lot of questions. As Marcus identifies it as a myth, it might not be true in all its details. Lurline is due to join the Olympian pantheon, but she steals items of power from the gods in order to enchant Oz, and is banished from Olympus. Exactly when this takes place is unclear, as there are references to both the fall of Troy and the three wise men following the star to Bethlehem, which happened over a millennium apart. It's definitely ancient, though, and it identifies the ruler at the time as Ozroar of Morrow. He's married to a woman named Ozia, daughter of Oziana. The first edition of this book made Ozroar and Ozia Pastoria's parents and Ozma's grandparents, but the later edition keeps things vague on this point.
I believe the king is also identified as Ozroar in Jeremy Steadman's Emerald Ring, which states that he threw the titular ring out to sea when he was dying.
Scott Dickerson's Magic Book tells how a fairy from Lurline's band named Ozymandias becomes the first ruler of Oz, and how he is turned into a wooden canary. Charles Phipps's Umbrella Man trilogy has Oz enchanted during the reign of a king named Saul, whose father left Oz in a magical boat, and who had an older brother named David who actually married Lurline. He is a terrible king until the Water of Oblivion changes his mind. My way of fitting much of this together is to say that Oziana was the ruler of the central part of Oz, and that his daughter Ozia married Roarer of Morrow. When Oziana died, Roarer became king of the central land as well, and hence took the name Ozroar. David and Saul could then have been his children.
We're also told in Magic Tapestry that Ozroar had an illegitimate son, Evrard, who went on to be King of Ev. Ozymandias complicates matters a bit, but perhaps he was Oziana's ancestor.
Adding in Phil Lewin's Witch Queen and Master Crafters is even more confusing, as it says that Lurline's older sister Enilrul was the original Queen of Oz, and Nikidik her consort. There's no reason all of these people couldn't have ruled at some time or other, but it's unlikely that the enchantment occurred in all of their reigns. Perhaps the records are unclear as to who the ruler was at the time of the enchantment, so historians just assigned it to the time of some old monarch or other.
The Water of Oblivion is another issue in and of itself. Emerald City has Ozma credit its creation to Glinda, but in Edward Einhorn's Paradox it's Lurline's work. Glinda is present at the time, however. Lewin makes the water's forgetful properties result from the fact that Enilrul committed suicide in a pool of water where the Forbidden Fountain is now located.
Paul Dana's Time Travelers restores the credit for the fountain to Glinda, but has Lurline spread its water throughout Oz to make everyone forget what happened before the enchantment. This tale makes the point that everyone alive at the time of the enchantment should still be there today, and vice versa. Certainly, if the description in Tin Woodman is accurate, this would largely be the case. Therefore, if several kings and queens have reigned since the enchantment, it couldn't have ended all death and aging.
As usual, it comes back to the likelihood being that there were at least two enchantments. One would have been at the time of an earlier monarch, or perhaps before there was a ruler of Oz, and would have set everything into motion. I don't like the idea that it created all magic in Oz, but I think it made the land more magical than the rest of the world.
The second enchantment would have been when Lurline left Ozma with Pastoria, and it's at this point that death and aging would have ended. Well, maybe. According to Paradox, aging didn't stop until near the beginning of Ozma's reign, due to the influence of the Man Who Lives Backwards. Either way, it was a relatively recent phenomenon nationwide, although it might already have existed in places like Samandra.