The age was actual because it is the Word of God.
Some would say it is the word of those who wrote it. For example in Numbers 22:28 it may say, "Then the LORD opened the donkey's mouth...", but I would contend it is not God who is saying "Then the LORD opened the donkey's mouth", but the human author of the text who wrote those words. Similarly in Genesis a human author is recording the old oral tradition that people of the past lived longer lives. I am only asking you to examine the cultural context that this text was composed within. The author of the king list recorded a tradition of their kings in the distant past having reign of many hundreds of years. The earlier dates are of many thousands of years, but closer to the contemporary author of the king list the age of the kings is comparable to the age of people in historical times. Did you see that the reign of one king was said to be 36,000 years! Examine the ages on the list.
Sumerian King List - Wikipedia, the free encyclopedia
In the beginning there was no disease and after the fall the creation was starting to die because of the sin that infested it. People lived a long time in the beginning but gradually due to disease, famine and other ills the average life span has been drastically reduced.
Anthropologists studying human remains from the past commonly find signs of trauma (from war) and disease. I am not going to take the time to find the sources but I have read that even some Neanderthal remains display evidence of disease (including arthritis, abscessed teeth, and so on). You are simply talking through your hat, as my mother would say. I would ask you to examine the anthropological evidence. There is absolutely no evidence that people of the past lived disease free lives of hundreds of years. If you think this is so then try to find the physical evidence. There is none, but there is plenty to show you that the opposite was true. People lived shorter lives in the distant past.