What does "life extension" really mean?

For a long time, I've been bothered by this puzzle...

We (and not just we) have shown in mice that adding just one substance can significantly ...extend lifespan.
(And we found many substances with such an effect).

But in reality, how "life extension" occurs, what "extended life by 10-20%" means - is not quite a simple topic. Because most often the ENTIRE mortality curve shifts to the right. That is, mice still die starting from an early age - but all "a little bit later". 😊
Let's say, instead of day 400, the first mice will die on day 500. Then instead of day 600 - on day 700... And so on. That is, mice on the substance will still die "on the curve"!They don't all live to a ripe old age! 🧑‍🦯

(I think most people in conversations about "life extension" overlook this point. Or they simply "don't realize it". And "intuitively" think that "life extension" is when "everyone lives long, to old age"! And no one dies "early".)))

So... For me it looks like a specific substance affects SOMETHING IDENTICAL at EVERY AGE!! Some FACTOR that at every age SOMEHOW affects the "date of death", determines it!!
👉 One possible version - for example - is that there's "some kind of POISON" (or toxin), which from early childhood all mice (and humans??) receive "with food or water or air"... And this poison poisons some faster, others - slower. And if you slightly "remove" it (with this substance) - everyone will live a bit longer!

I also think that, possibly, "death" occurs as a result of a certain "set of factors". For example, there are some 10 factors of "death", like a "layered cake". And a specific substance can affect some one factor - and this leads to life extension by 10% - at every age! But other factors remain, so death still occurs)))
But this is somewhat beside the point)))
I tried to discuss this with Ryazanov, but with him everything immediately came down to "death probabilities", "randomness", "statistical errors")))
In general - I continue to search for hypotheses!! +++++++++++++++++++++++++++++++++++++++++++++++++++++++++

Below (1) is roughly how some substances in our experiment shifted the mortality curve of mice. (Rapamycin has a similar curve shift).

And this is (2) roughly how total mortality is distributed in humans. This is how life expectancy "increased"!! starting from 1900 - in "every age - slightly to the right"!! More and more people live a little longer. Not very many early deaths - but they exist too. Main mortality - around "average life expectancy" (although the spread here is from 65 to 95 years - that is 30 years!! And why someone lives to 65, and someone to 95 - we have no answer! Although there are many myths - explanations when someone dies))) "Drank, smoked, was nervous, heart problems, didn't exercise" and many other things... But the truth is probably completely different!!)