I can agree with natural life extension ideas such as taking supplements of nutrients that we lose the ability to digest as we age (that are linked to the aging process). My problem, however, comes in when we consider the more extensive methods of life-extension, for example mechanical or nano-technological enhancements, genetic engineering and so on.

To begin, we have neurons not just in our brain but spread everywhere throughout our body, with particularly dense concentrations in our gut and heart. These gut and heart "brains" interact in a complex manner with our brain, with both top-down and bottom-up influence occurring (Source 1). When we take this information together with the fact that electrodes connected to the brain can control the brain (Source 2), it is not clear, as mechanical enhancements become smarter, that the only influence will be top-down. In other words, while one may influence the enhancement, one is likely to also be influenced by the enhancement. This doesn't even take into account the potential for purposeful abuse of this neuronal connection to the brain by the corporation or government that is making the enhancements (or a hacker).

Nanotechnology also holds vast potential for abuse as it would potentially give the creators control over human bodies at a cellular level. In the case that the technology were not so controllable (more automatic) then there is the issue that the nano-tech may run amok by itself, acting as a pathogen. This second case actually makes a lot of sense considering that one's immune system can be triggered merely by common food proteins and nano-tech is often self-replicating.

As for the genetic engineering of humans, this is a bad idea for a number of reasons. First of all, I would imagine a "Brave New World" scenario, where there are several different grades of human. The reason for this is because is everyone were (for example) a leader then society couldn't function. As we've already discussed elsewhere at length, creating a ruling class leads inevitably to the abuse of any underclasses.

Alternatively, I'd imagine a "Gattaca" type society (another dystopia), where everyone can choose their child's genetics. However, due to the fact that nobody would choose certain genes (for example tall genes would always be preferred over short ones) over time humanity would become more genetically homogeneous. Aside from the increased risk from pathogens, we would also be less diverse in our thinking, behavior and interests, as temperament (personality) is in part genetically determined (Source 3). This, obviously, would stagger our progress as a species. Further, different genes give advantages in different situations and as such we would be reducing humanity's ability to deal with unforeseen problems. Moreover, such trait selection could also cause genetic diseases several generations in the future that cannot be foreseen now. Finally, it's possible that certain genes (such as ones causing greater submissiveness) would be forced on people, under the pretext that such genes are better for society.

Sources:

(1) https://en.wikipedia.org/wiki/Gut-brainaxis

(2) https://www.nytimes.com/2017/06/01/health/new-electrical-brain-stimulation-technique-shows-promise-in-mice.html

(3) https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1188235/