On this day, the sunlit graveyard belonged to the robins.

The harbingers of new life soared and swooped through the kingdom of death at North Side Cemetery in Butler, Pa., drawn more by the freshly mown grass than by the quiet stone markers.

One of the birds alighted briefly in the patch of green between two stones, the first shaped like a gothic spire, the second like a heart on a pedestal. The spire marker read, “Bertha A. Heck, wife of J.H. Kepple, Aug. 3, 1873-Jan. 4, 1904.” In its shadow was the heart marker: “Howard, son of JH, BA Kepple, Jan. 27, 1895-Nov. 21, 1903.”

In their mute way, these 101-year-old gravestones tell the story of life expectancy a century ago and how it has changed since then. They reveal facts that may be surprising to some, and they even say something about how long we are likely to live in the 21st century.

The main lesson is that the “average life expectancy” we might have thought we understood is actually more complicated than it appears.

That’s partly because the term has been mistaught in schools for years.

A standard world history textbook being used today in many classrooms is a case in point. In one chapter of Glencoe/McGraw-Hill’s “World History: the Human Experience,” dealing with the improvement in living conditions in the United States, the textbook says that “in 1850, the average person lived about 40 years. By 1900 most people could expect to live beyond 50.”

Right numbers, wrong phrasing.

In both cases, the textbook is talking about average life expectancy, or more technically, “period life expectancy at birth,” which is not the same as “how long the average person lived” — at least not in common-sense terms.

S. Jay Olshansky, an expert on aging and a public health professor at the University of Illinois at Chicago, said the average life expectancy figure was “often misinterpreted and misused as being the average age of death in any particular era.”

But it’s actually the average of all life spans for those born in a particular year. So in 1903, when childhood deaths from epidemics like typhoid were still too common, the larger numbers of children who died decreased average life expectancy compared with today.

So, while it’s true that adults are living longer today than one or two centuries ago, the number of extra years we get is not as dramatic as the history textbook might make it appear.

This longer life span for people who make it to adulthood is the reason why statements like the one in the “World History” textbook can be misleading. When it says that “in 1850, the average person lived about 40 years,” it is using the average life span at birth. But even in that year — a half-century before the Butler typhoid epidemic — the average life span for a 20-year-old man was 59.

This helps explain why previous eras have so many well-known people who lived to ripe old ages.

Just a smattering of famous names makes the point:

St. Augustine, who lived in the fifth century, died at 75. Despite a life of marauding and pillage, Genghis Khan, who lived in the 12th century, survived to age 63. There’s a problem with spotlighting such famous personalities, though, Olshansky points out.

“When you’re talking about famous people in history, they were famous partly because they were able to live as long and be productive for as long as they did,” he said, “so they don’t constitute an average group.”