There are two different answers here, and they come from two different kinds of evidence. The Bible records a handful of very long individual lifespans before the Flood — Genesis 5:27 says “Methuselah lived a total of 969 years, and then he died.” Historians and archaeologists, working from skeletal remains and population records rather than Scripture, put the typical lifespan across the ancient world far lower. Navigate to this website to learn more about the biblical question.
Definition of “Before Christ”
“Before Christ” (BC) is a calendar label, not a description of one biological era with one typical lifespan. It simply marks the years before the traditional birth date of Jesus on the Western calendar, and it covers many thousands of years and civilizations whose lifespans differed enormously from each other. Navigate to these guys for detailed information on the difference between life expectancy and longevity.
Average Lifespans
According to Wikipedia’s overview of the historical record, human remains from the early Bronze Age suggest a life expectancy at birth of about 24 years, Classical Greece was around 25–28 years, and Ancient Rome was roughly 20–33 years. Those low numbers are driven mainly by how many children died young, not by every adult dying young: about half of all children died before adolescence, but a person who survived to age 20 in Ancient Rome could expect to live around 30 more years, and someone who survived to age 15 in Classical Greece could expect to reach 37 to 41. In short, life before Christ was cut short far more often by childhood mortality than most people realize, and adults who made it past childhood commonly lived into their 50s or 60s.
Factors Affecting Lifespan
There are a number of factors that affect lifespan, including genetics, environment, and lifestyle.
Disease
Disease and lack of modern medicine were major factors in how short many ancient lives were, especially for infants and young children.