Suppose there was no senescence, we all lived perfectly healthy and at peak capacity for the duration of our lives. We would still have limited lifespan in nature, because of diseases, natural predators, etc. There would be a life expectancy, since people would not live forever. Now suppose a mutation occurs that grants the individual a boost in body functions at a young age with a caveat: a gradual decline in body functions after a certain age. If this certain age is way past the life expectancy of the population, this mutation would give competitive advantage to its bearers, with no real downside, since nobody lives way past the life expectancy anyway. This mutation would spread throughout the entire population and aging would establish itself in this population.
The point of equilibrium in these trade-offs would be different for each population, since the environments and species are different, hence why hummingbirds, dogs, humans and turtles have wildly varying life expectancies.
Trying to stop aging would involve stopping or mitigating these mechanisms, which are deeply ingrained in our systems. The consequences of doing so are far from straightforward, both to the individual (if we could not stop side effects to have the cake and eat it too) and to the population as a whole.