Aside from textual difficulties (there are many differences between the most ancient texts on this matter) we are faced with the question of how to explain these exceptionally long life spans (all these individuals were at least 65 when their first child was born, and most lived for up to 1,000 years!).  The problem is compounded by the fact that if we put the various data together, we come up with a date for the creation of Adam (about 4004 BC) which would be even more recent if we assumed normal life spans for his descendants!

Various theories have been put forward.

1. the traditional view is that the numbers are to be taken literally, without gaps, and that the earth is, accordingly, just a few thousand years old.  Howe and Geisler (When Critics Ask) offer the following points in favour: ‘(1) First of all, life was later shortened to 120 years as a punishment from God (Gen. 6:3). (2) Life span decreased gradually after the flood from the 900s (Gen. 5) to the 600s (Shem 11:10–11), to the 400s (Salah 11:14–15), to the 200s (Rue 11:20–21). (3) Biologically, there is no reason humans could not live hundreds of years. Scientists are more baffled by aging and death than by longevity. (4) The Bible is not alone in speaking of hundreds of years life spans among ancients. There are also records from ancient Greek and Egyptian times that speak of humans living hundreds of years.’
2. As a variation on the first option, Kaiser (Hard Sayings of the Bible) thinks that the ages Adam and others are to be understood literally.  The long life spans reflect ‘the fact that the effects of the Fall into sin had not yet affected human generative powers as seriously as they have more recently’.  However, he takes an ‘age-day’ view of Genesis 1, and thinks that the sons attributed to Adam and others may have been between one and six generations away.  (In support of this latter point, Kaiser points to Gen 46:18, where Zilpah is said to ‘bear’ her grandchildren, and Gen 46:25, where the same is said of Bilhah).  Kidner takes a similar view.
3. the years of Genesis 5 were much shorter than ours.  Perhaps we should understand them as months, rather than years.  Adam’s age of 930 years would then become a more manageable 80-odd years.  The trouble is that according to this theory Nahor would have fathered Terah at the age of two, rather than 29 (Gen 11:24)!
4. there were long gaps in the genealogy (so W.H. Green)
5. the numbers indicate the lifespan of the family, rather than the lifespan of the individual (Kidner).  But this would not work for Enoch, of course, who was ‘taken up to heaven’.  Howe and Geisler state a number of valid objections to this interpretation: ‘First, some of these names (e.g., Adam, Seth, Enoch, Noah) are definitely individuals whose lives are narrated in the text (Gen. 1–9). Second, family lines do not “beget” family lines by different names. Third, neither do family lines “die,” as each of these individuals did (cf. 5:5, 8, 11, etc.). Fourth, the reference to having “sons and daughters” (5:4) does not fit the clan theory.’
6. a different numerical system (e.g. sexagesimal rather than decimal) is used here.
7. the ages relate to various astronomical periods (Barnouin).
8. the long life spans reflect theological (and possibly not historical) truth: not only procreation, but long life, is seen as a blessing and hope from God.  Mathews writes: ‘In the Mosaic law long life was the product of God’s blessing for obedience. This was etched in the mind of the community by the Fifth Commandment (Exod 20:12; Deut 5:16), which is distinctive as the first of the commands with a promise (Eph 6:2–3). Long life was commonly tied to the heritage of living in the land (e.g., Deut 4:25; 30:20). Outstanding heroes, such as Abraham, Gideon, and David, were said to have lived to a “good old age” (Gen 25:8; Judg 8:32; 1 Chr 29:28). Strikingly, apart from the patriarchs of Genesis, in the Old Testament only Job (140), Moses (120), Joshua (110), and Jehoiada (130) lived longer than a century of years. Isaiah also points to long life as a feature of blessing in the eschatological age (Isa 65:20).’
9. Wenham, after reviewing various options, concludes that no adequate explanation is available at present.  McKnight (DOT:P, art. ‘Seth’) notes: ‘there are no successful attempts at explaining the ages of the antediluvians other than that each one met the same end.’