Scientists find humans age dramatically in two bursts – at 44, then 60

This lines up quite well with the late-acting error theory of aging. This theory basically states that many mutations have some kind of clock associated with them, ie they are more likely to cause negative effects after some period of time. There is a lot of evolutionary selective pressure to weed out such mutations that trigger when we are young as those impede reproduction. However as we get older, there is less selective pressure, particularly in pre-industrial times. There is still some selective pressure, as living parents and grandparents help raise and provide resources for later generations even after they stop reproducing, but it diminishes.

So a mutation that causes heart attacks in your twenties is obviously going to die out, but a mutation that causes heart attacks in your 50s could survive. If you had a mutation that would guarantee a heart attack in your 130s, you would never know, nor would evolution; but as the odds of living to an advanced age increase, so too do the odds of experiencing the negative effects of these late-acting mutations.

If this theory is correct, we should not see a perfectly smooth increase in age-related effects, instead we should see spikes near significant life milestones. Assuming a generation length of about 20 years, these peaks correspond reasonably well to the points where grandchildren would be able to take care of themselves, and the point where those grandchildren start having kids of their own. The rate of change of your biomolecules is the derivative of the selective pressure. So basically there is strong selective pressure to live long enough to help with the early days of raising your grandkids, then pressure suddenly drops to an intermediate level - a grandparent is about as helpful to a 8 year old as a 14 year old - then by the time you're a great-grandparent your progeny is setup as good as its ever going to be, and selective pressure drops further.

While I am sure there are ways of explaining these peaks with other theories of aging, like lifestyle changes at the ages increasing the rate of accumulation of damage, it seems to me such explanations would not be resilient across genders and especially not across cultures or socioeconomic conditions.

The figure showing the bursts is at [1], and the Nature article itself is at [2].

All the people in the study lived in California, perhaps near Stanford where many of the authors are. It would be interesting to repeat the exact same process on a completely different cohort, say a South American tribe. Or for that matter, farm workers from California's Central valley.

[1] https://www.nature.com/articles/s43587-024-00692-2/figures/4

[2] https://www.nature.com/articles/s43587-024-00692-2

If this is true, shouldn’t we see small spikes of mortality at those ages?

Observationally and personally true.

honest to goodness, I think this lines up with my experience, at least for men.

Not sure about menopause lol!

Honestly 40 has been pretty rough!