In the horror genre there are supercentenarian humans or humanoid monsters who maintain virtual immortality by sucking the vital juices out of the young–or purloining their organs. In some versions this is a cyclical process in which they need to rejuvenate every few years. They have a narrow window of opportunity, and become increasingly frantic. Due to the conventional Christian morality of the genre, they typically come to a bad end. When their effort to find fresh young victims to suck dry (or harvest organs) is thwarted, they age centuries in a few minutes.
But what used to be horrific fiction is becoming horrific reality. You have the recent story of Planned Parenthood hawking baby corpses.
You also have a movement to harvest the organs of patients declared to be in a persistent vegetative state. This despite the fact that some comatose patients wake up years later, with mind and memories intact.
In addition, if the society becomes increasingly secularized, some rich folks might endeavor to extend their lifespan having their vital organs replaced with organs harvested from healthy young men. There are parts of the world where you can procure anything if you have enough money. Just bribe the authorities to look the other way.
Of course, you can't replace the brain, but perhaps having newer organs would retard the aging process of the brain.
To be sure, there are medical complications, which are magnified by multiple organ transplants. But desperate people do desperate things.