[phreeza]

I was simplifying, the point is that the model should always reflect the current state of the population. You shouldn't expect the kind of "catch up" effects that OP is referring to, unless you have static predictions that don't take into account actual deaths and births.

[klmadfejno]

But... you would. If all elderly people dropped dead right now, deaths per year would plummet for decades.

[phreeza]

And your model would reflect that after the day all the old people died. So you wouldn't see any negative excess deaths (= actual deaths - predicted deaths). Otherwise you would see massive excess deaths from the baby boomers getting old for example, or negative excess deaths from the WW2 generation in places where many young people died.