From considering some of the points in this post
it is clear that the ageing population, the change in the age structure (as represented by a population pyramid) has been a factor in reducing average intelligence.
Almost all nations at present have grossly distorted population structures resembling one of another of these extremes:
In terms of the median (average) age, Angola is in the late-teens, Japan is in the mid-forties.
Such extremes of median age have not been seen in human history, and such an extreme difference between nations is highly significant.
The population structure ideal is something in between, and closer to the 'stationary' shape - therefore probably it would be roughly 'pyramidal', but with a much narrower base than Angola.
The developed world nations all approximate to the top-heavy Japanese shape among the indigenous population - very few children at the base of larger proportions of the elderly.
As a person ages they suffer a decline in intelligence - which is objectively (but only approximately) measurable by a slowing of simple reaction times.
Reaction times get faster from young childhood up to sexual maturity as intelligence increases throughout childhood and up to about age 16 for girls and 18 for boys.
Adults are more intelligent than children - and a population grossly-over-dominated by children and early teenagers, like that of many of the developed world nations, will therefore natural have a lower average intelligence. However, such populations would expect to get more intelligent over the short to medium term (forthcoming years to decades) as current children mature into adulthood.
The opposite applies in the developed nations.
The reaction times/ intelligence does not change much during early adult life; but seemingly decline gets faster and faster in the thirties or forties; so the decline in intelligence from age forty to fifty is much greater than from thirty to forty, and continues to accelerate.
(The actual amount of decline is only imprecisely known, I think, because it would require longitudinal studies lasting many decades. But as a very approximate ballpark figure, I would suggest a loss of about 10 IQ points (2/3 of a standard deviation) from age 20 to 70. Therefore the decline would go something like 30-40 - 1 IQ point lost; 40-50 - 2 IQ points lost; 50-60 - 3 IQ points lost and 60-70 - 4 IQ points lost.)
The developed countries currently have a median average age in the mid-forties, which means that average intelligence has already declined - but as the median age gets above 45 and continues to rise, the rate of intelligence decline will increase further, and further.
At a national level, there would appear to be an apparently sudden, because more rapid, and unavoidable decline in national capability to accomplish functions requiring a population of high intelligence.
Of course, intelligence is not the only thing that changes with age - physical ability declines, and personality also changes - but the objective nature of reaction times makes the picture simpler and more objectively measurable, in principle.
(Objectively measurable, that is, at the population level where the imprecision of simple reaction times for estimating individual intelligence, is overcome by averaging of larger numbers.)
The point is that a population with a top-heavy population pyramid, a population with a median age in the forties and increasing, is a population with:
1. Reduced average intelligence compared with the optimal population structure - and the transition of population structure to the top-heavy form would be accompanied by an increasingly rapid reduction in average intelligence from this cause;
2. And a population with high median age/ top-heavy structure is a population where further and more rapid decline in average intelligence is to be expected over the short to medium term (the coming years, and next few decades; due to the small proportion of the population contained in those age- cohorts that will be moving into young adulthood (with peak intelligence) in the near future.
Note: Of course, the Leftist media propound sustained mass immigration as a solution to this problem; but of course it is not a solution to this problem and very obviously leads to multiple other and intractable problems.
Suffice to say, any potential immigrant groups that could theoretically improve the cognitive deficit will not improve (and probably worsen) the reproductive deficit - and vice versa.
In the long term, the current top-heavy population structure of the developed nations will self-correct, because it is unsustainable in multiple ways; and thus the ageing-contribution to intelligence decline will cease.
However, the intelligence level of the population which stabilizes will, of course, be significantly lower than it was 150-200 years ago - due to the substantial intelligence decline over that period.