Past perfect tense understand use

Question

The following paragraphs describe a chart.

Although there was no change in the number of people aged 65-74 between 1960 and 1970, the number of people aged 75 and over had increased during this time. By the year 200, 12.% of the US population had reached the age of 65 or more, although this was slightly lower than in 1990 when it had peaked at 12.6%

The chart shows that today people in the United States can expect to live longer than in 1900. By the year 200 more than 12% of the population had managed to live to the age of 65 and over compared to only 4.1% in 1900.

I have some doubts with past perfect that are used in this two paragraph.

By definition, When we are talking about past and want to mention something that happened earlier we use past perfect tense.

This rule seems not applicable for the above example. Then why we use past pee]rfect without using simple past tense?

Answer 1

If the graphs were plotted in the past to decribe those changes in population trend that had happened, we use past perfect. I believe this passage is based on that.

On the other hand, if we are plotting the graphs now, we use simple past tense to describe those changes in population trend.

Answer 2

Each of these past perfect sentences is viewing the data as if from a point in the past. In the case of the second and third sentences the viewpoint is the year 2000 (written 200). The past perfect tense is appropriate

The first sentence is probably viewing the data from a viewpoint in 1970, but the observation would be equally valid from any viewpoint. It could have been written in simple past tense, but past perfect is still OK. It is likely that past perfect was used here for consistency of style.