The ages mentioned in Gen. 5:1-32 range from almost a thousand years to Enoch's life at 365 years. Scholars have yet to come up with a completely satisfactory explanation for these large numbers. Some have suggested that the years were shorter than ours. But the timing described in the flood story makes it clear that a year was about 350 days. Not much difference there. And if the years were different, then Adam would be less than 4000BCE. Others claim that the list is not complete, that many generations were missing. Perhaps, but it states rather clearly how long a patriarch lived after his son was born. Then there are those who compare the lists to those of other ancient cultures, noting that eight Sumerian kings reigned for a period of 241,200 years. The sum total of the patriarchs is only about 6700 years -- short, by those standards. To date, scholars admit that if these numbers are symbolic, they haven't figured out the symbolism yet. Perhaps the numbers merely show how being made in God's image played out over the years. The fact is that the patriarchs were, indeed, fruitful, and they multiplied. And finally maybe it was just the authors' way of indicating it all happened a LONG time ago.

Source:
Wenham, Gordon. "Genesis 1-15." Word Biblical Commentary, Genesis. Waco, TX: Word Books, 1987, 133-34.