Various attempts have been made to square the ages recorded in the early chapters of Genesis with a literal reading. Theologians have suggested that first generations of humanity were more perfect physically due to being so close to the initial fall of man and that over time, as more sin and corruption set in, so too did humanity physically decline.
Other theologians and scholars note that ages used in stories of the ancient world were not meant to be taken literally. They were of symbolic value. For example, the ancient tablet listing the chronology of Sumerian kings declared various kings reigned for 28,000 years, 43,000 years, and 36,000 years. Outrageously long reigns were most likely used to convey the strength, power, and legitimacy of the ruler.
In the modern world, if we saw a list of significant people and all were listed as having died in round number ages (70, 80, 90, etc.) we would instinctively know that it was highly unlikely that everyone on the list died in years of ten and that the age listed was an approximation. In a similar way, many of the prominent ages reported in Genesis (Adam, Methuselah, Noah) are multiples of 19.
Another example is that Joseph and Joshua are both reported to have lived to 110 (Gen. 50:22, Jos. 24:29). Both lived considerable time in Egypt, where 110 was considered the fullness of a life lived wisely.
The common understanding is that these ages had symbolic value in the times in which they were written. In this line of thought, there is no need to justify the possibility of such long life spans, because the author never intended for them to be taken literally.
What exactly these numbers signify is greatly debated by theologians and scholars.