When we read numbers (0,1,2,3,4,5,6,7,8,9) in Holy Writ we sometimes fail to realize that those numbers didn't exist until well into the Common Era. In both the Old Testament and the New Testament letters served as numbers as well, so there is the probability that what was translated as a number in our Bibles may have been intended in a different vein.
An example of this is the number '40'. Scripture is replete with that number. Moses fled Egypt, and was away for 40 years. Moses led the Hebrews in The Wilderness of Sinai for 40 years. Jesus was led into the wilderness of Judea for 40 days to be tested by Satan. The disciples waited in Jerusalem after Christ's ascension for 40 days until Pentecost. Even the time which the Hebrews spent in slavery in Egypt is accepted by many as being 400 years (a derivative of '40').
However, I believe that the number '40' (and '400') was intended to serve another purpose. Ancient Hebrew was limited in its vocabulary, as were Greek and Latin, so the scribes used numbers to express an appropriate length of time in many cases, rather than using them as a literal timeline. Under this definition Moses fled Egypt until the time when God knew that he was prepared to return. Moses led the Hebrews through The Wilderness of Sinai until the people had finished their military training and learned the laws of Torah, so that they could go into Canaan as a united force to conquer it. Jesus was led into the wilderness of Judea and tested there until it was clear that he was prepared for the task that lay ahead of him. Even the disciples were ordered to do nothing until the events which they had witnessed had time to 'sink in'. In each instance the 'numbers' could also mean 'the appropriate length of time'.
In like manner the ages of those men in prehistory in all likelihood meant something different to the original readers of Genesis. They had lived the appropriate number of years that enabled them to achieve what God intended for them to achieve. How long they actually lived we will never know, nor is it important. What is important is that they lived long enough to accomplish all that God wanted of them, after which they died.
