The basis of my idea comes from what Diez Barrels says about the fruit in this panel right here.
This panel tells us that a great doctor once ate it and became famous for his/her abilities. We also know of a certain place that is said to be famous for its medical science! That place is Drum Island. The thought that started this whole theory was that what if the start of Drum Island as a medical country, and the reason it is still known as such, was the famous doctor from the Barrel’s panel. Maybe others flocked there to learn from this famous miracle doctor, and this is why the country is so medically advanced?
That is where Kureha comes in. My idea is that ages ago she came to the island to learn from, or maybe be healed by, this famous doctor. Either way, she somehow ended up studying under him/her or spending a lot of time there, which partially explains her exceptional medical prowess today. Maybe Kureha even had an age related disease that even the miracle doctor could not heal, so he/she decided to give his/her life for Kureha’s which would give added weight to her involvment in Chopper’s backstory (Though contradicting this moment).
Her/his death could have been further motivating Kureha to be an even better doctor. Or maybe the doctor was about to die of an incurable disease, and decided she/he should pass on his medical knowledge to Kureha because she/he was going to die anyways. Something of the sort, though the first seems more thematically likely.
The final piece of evidence is her famous catchphrase.
Sounds silly, I know, but it is a bit interestingly suspicious that there is a lady famous for medical skills that has a “secret of her youth”. When there exists a secret way for a skilled doctor to give someone eternal youth. Worth noting is also that Kureha is one of the few people who has not changed at all in her design over the timeskip, she just switched clothes. It’s almost like her physical state is frozen in time…
That’s basically it, my whole idea.
*Theory by 4rca9