Aging is a complex and barely understood process. While some animals have lifespans of centuries, others live for mere months. It is unclear why.
We know that what we call aging involves multiple cross-linked systems. Here's De Grey's breakdown of different types of aging damage from SENS:
- Mutations in chromosomes
- Mutations in mitochondria.
- Intracellular Junk
- Extracellular Junk
- Cellular Loss
- Cell senescence
- Extracellular protein crosslinks
As you can see, this is a wide-ranging field, making it difficult to argue that a single (complex) molecule complex could address all 7. More likely, an anti-aging treatment would be customized for each individual (thus expensive at start) and would consist of a complex cocktail of active agents.
Using someone else's cocktail might yield anything from near-equal benefits to deadly metastatic cancer within days.
The standard techno-utopian answer is that all sorts of miracles will become possible with nanotechnology: we will simply build nanobots to wander our bloodstream and clean up stuff. The problem with that is that at nanoscales, stuff gets squishy and sticky. Without their own repair mechanisms, nanobots are likely to malfunction and could end up doing more harm than good after a while. Moreover, without a way to centralize information and determine a path forward strategically, said nanobots would be unable to determine a best path forward.
This is why a realistic answer (from the perspective of our current technological frontier) involves an ever-shifting cocktail designed specifically for the individual, specifically for the exact circumstances their body is in at the time. One size likely will not fit all.