There was an increase in the percentage of aging population in Japan, Sweden
and the USA.
解析：通常 in +时间(如in 1940)/地点(国家、城市等);当然，在没有给出描述对象的句子中，可以用in引出描述对象
There was a general upward trend in the number of elderly people in all
three countries over the period from 1940 to 2020.
There was a slight rise from 8% in 1940 to 10% in 1960 in the percentage of
elderly people in the USA.
解析：from
…to固定搭配，常常结对出现，表示“从…到…”，体现动态变化;但若上文说过起点数据，则直接写to+终点数据即可，表示“达到某个值”。
There was a sharp increase of /by 5% in the percentage of elderly people in
1980 in the USA.
解析：of +数据，表示变化的差值;by+数据，同样也表示变化的净增量或净减量，用来体现动态变化。
