What I never understood is that, for the most part, a 30-something year old male and female can create a pristine new being from their already-slightly-knackered cells. I know it breaks down somewhat with age.

So... we can create pristine new life with biological clock reset at 0, but not do so for our own bodies.

Women are born with their supply of eggs, so they are more ‘pristine’ than any other cell. Sperm are generated continuously throughout life, and there is evidence sperm quality decreases over time. Some diseases associate with increasing age of the father (autism, schizophrenia). The effect takes hold mainly after paternal age 40. Having said that, I recall there is some work showing sperm generating cells are kinda special in that they don’t develop as much mutational damage as other cells. Does make you worry though that progressive generations of older and older fathers may lead to some kind of problem. With rates of assisted fertility increasing, perhaps 16 yo boys should bank sperm for the future.

Its by design, you are meant to pass on the genes and die. Thats how nature works.

Thats the best mechanism nature have to make sure we are evolving - more precisely mixing genes.

What people forget is that our enemies are constantly evolving and trying to kill us, bacteria, viruses, fungi etc. Passing on you dna and eventually dying frees up resources for your offspring to do the same, creating new combination of genes increasing odds of more optimal defense for current threats.

If lions didn't age. They would compete for same patch of land and given all are at the peak of their development roughly 50% of old genes would win resources. Slowing down gene mixing and genetic diversity.

Actually speaking of big cats. Look up cheetah and what happens when you don't mix genes often enough (for natural or unnatural reason)

"Cheetahs retain only 0.1–4% of overall genetic variation seen in most living species, much lower than other well-known examples of genetic impoverishment including"

That's why we have babies at all instead of just budding new copies of ourselves. Compressing everything down to gametes is a powerful form of error correction because it reduces degrees of freedom so heavily.

Sounds like a feature, not a bug. Unqualified speculation incoming, but I suspect that lifespans for different species approach the optimal replacement cadence for maximum fitness.

Women are born with all their eggs, which do deteriorate. The vast majority won't make it all the way through puberty even (lots will be gone by the time they're actually born). That's why there's a drop-off in fertility over time.

I guess having individuals with unbounded lifespans would slow down a species' evolutionary rate (less pressure to procreate), that would be a disadvantage.

Having just gone through it, what I found surprising was that cats one one hand are less likely to develop cancer than dogs, but also to develop types that are harder to treat and develop much more aggressively.. all of which leads to it being seen as practically untreatable in most cases.