Evolution doesn't try to head towards any global optima, so it is quite believable that certain things just don't work a certain way even if they sound 'obvious' to rational beings.
If one even exists. I don't think you could actually say a global optima exists in a lot of cases. Since there are a lot of compounding factors and the functions aren't required to be continuous.
Of course, old people can learn new things so I always thought the "neurons don't grow" thing was silly (although I guess it's possible that we have a set number of neurons and it's just our synapses that keep growing).
Those people who loose saliva production end up loosing teeth as well, because the enamel disappears.
I have no idea about corneas.
Apparently whatever mutation happened in the past in this regard was not good enough / human still didn't survive, or it didn't happen yet. It might be funny if we discover some remote tribe actually has this regenerative mutation.
I think its question of time (maybe very long time), look at our unique ability to process lactose in adult age.
This is definitely wrong. Individuals (e.g. grandparents or just random society members) can influence survival rates of other society members.
Some species(e.g. ants) even have sterile members that definitely exert evolutionary pressure.
Sterile ants do not exert evolutionary pressure unless they would attack weak queens before they are ready to reproduce. Evolutionary pressure (by scientific definition) is caused by things that reduce reproductive success in individuals or parts of population. Like a mutation that reduces fertility. Having the old individuals around might help relieve some of the pressure by infinitesimal amounts (helping raise offsprings) but that's about it. They cannot influence the outcome of a "defective" gene so they do not really drive evolution in any meaningful way.
Take this example. A genetic disorder that causes blindness soon after birth basically eliminates the individual from the population and the genetic defect along with it. Thus evolutionary pressure (genetic defect before the reproductive age) achieves the selection.
A genetic disorder that causes blindness at old age, any time after the next generation was raised, might also eliminate the individual but this doesn't matter as much. The gene already got passed along to the next generation(s). As long as it manifests only after reproduction then it does not reduce reproductive success, so exerts no pressure.
Or perhaps you want to take something like male impotence as an example. If humans were expected to bear offspring only in our 70s I'm certain Viagra wouldn't be a thing. Only the males that are still able to perform at that age pass along their genes, to the point where the males that can't are the exception.
I hope you can appreciate the difference in magnitude (many orders) between directly applying evolutionary pressure by removing the individual from the gene pool before the gene can be transmitted, and indirect evolutionary pressure via reduced support in society (which could conceivably affect reproductive success). For one, it stands to reason that solitary species or ones that do not form real societies would be completely unaffected by this indirect evolutionary pressure.
Sciatica doesn't generally show up until people are in their 40s-50s. This is long after childbearing years for most people. At that point your sciatic nerve has done its job and gotten you across the finish line. What happens next is less important.
Isn't that given modern nutrition, hygiene, water access, tooth cleaning etc?
The other commenter said that people that survived childhood during these times usually were expected to live around 54 years.
1) looking at people before and after 1 month of regular exercise.
2) healing of heart after large and invasive surgery.
I had some nerve damage a while back and went to a neurologist. He did an EMG test (https://en.wikipedia.org/wiki/Electromyography) and confirmed the nerve was damaged. He used electrodes to show that it did not conduct electricity like it was supposed to, nor did it conduct electricity like the corresponding one on the other side of my body. (Somewhere, I have a printout showing voltage and latency numbers.)
He then told me that if the damage is not too great, nerves can repair themselves, that the repair proceeds outward from the brain toward the periphery of the body, and that it happens at a rate of roughly 1 millimeter per day. So, he said, my problem would probably resolve itself but it would be a long time, think 6 months or a year, maybe even longer to finally be back to 100%.
And it basically happened just like he said it would. 6 months later, it was a lot better, and it continued to gradually improve from there.
He also mentioned that using the affected muscles would help encourage healing. I think he meant the nerve itself would heal faster this way, not just that it would help reverse muscle atrophy.
He said that if the outer layer is intact, the inner layer can be repaired. I'm not sure if that means more nerve fibers form or that existing nerve fibers are repaired but that the repair takes place incrementally instead of all at once. Intuitively it seems to make sense that a new fiber is growing, but I'm wary of speculating based on what seems to make sense to me.
We do not want nerves to regenerate in most cases because it could cause even worse results. If you start regrowing a batch of nerve, you have a chance of growing out nerves that transfer pain signals into places you don't want them to be, which could cause persistent neuropathic pain (https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5290455/).
The case in point is the sciatic nerve is massive, and prone to constant stresses and strains, yet supposedly never regenerates.