[Serious] Stats people: can you tell me why there's fewer 45-49 year olds than say, 50-54 year olds, or 40-44 year olds?

Not 100% - but there's such a thing as a war 'shadow' in that, where there is a situation of population decline due to an event- say- war, famine, natural disaster- supproessing births for say 5-10 years, there tends to be a 'shadow' of this like 25 years later. This is the replicated in the next generation, as there are less fertile aged women, and to a lesser extent men. Because there were less born during the bad times 20- 30years ago. Looking at this pyramid, this might be evident in the 65-70 age group (war, or even a shadow from the previous war on top). The 45-49year odds, and 20-24years - my basic point being their isn't just one dip- it repeats like the narrow bits in the caterpillar. It's the repeating effect of something- maybe the war and rations period?