The primary improvement to human life expectancy was not medicine, but public sanitation. Prior to that, death was bi-modal, with peaks at infancy and old age. When people (and doctors) started washing their hands, the infant peak was flattened. Dead toddlers really kick the average in the nuts.
There is a strong correlation (noting my word choice here before the incorrectors accuse me of confusing it with causation) between general anesthesia and onset of dementia. I’m not going to dig it up, but it is easy enough to find on pubmed. There is a decent amount of noticing in the literature.
The human body is a pretty amazing construction, nature doesn't make a lot of mistakes.