Why are the men of Genesis so old? Were the dating systems just different? Are all of these old ages supposed to have symbolic meaning?

There is nothing symbolic about those old ages they are there to remind us of just how continuing to sin sorrows God:

All Scripture is quoted from the King James translation.

Gen 6:1 through 6

And it came to pass, when men began to multiply on the face of the earth, and daughters were born unto them, that the sons of God saw the daughters of men that they were fair; and they took them wives of all which they chose. And the LORD said, My spirit shall not always strive with man, for that he also is flesh: yet his days shall be an hundred and twenty years. There were giants in the earth in those days; and also after that, when the sons of God came in unto the daughters of men, and they bare children to them, the same became mighty men which were of old, men of renown. And GOD saw that the wickedness of man was great in the earth, and that every imagination of the thoughts of his heart was only evil continually. And it repented the LORD that he had made man on the earth, and it grieved him at his heart

Adam was created to live forever. However, Because of the fall, he had to die. He also forfeited his right to eternal life. The genetics of Adam were such that generations following would benefit exponentially, until the influence would be depleted. Thus, succeeding generations lived shorter periods of time. The ages provide historical information. However, more importantly, is the lineage. Somwhere in this lineage the seed would come, Jesus Christ.

Dittos to Narnian's answer, I won't repeat his material, but let me add some further thoughts:

I've heard theories that the word translated "year" in our Bibles is really referring to some much shorter unit of time. One writer suggested it should be translated "month", so the ages of 800 and 900 years become 800 and 900 months, or 70 or 80 years, and thus sound much more reasonable.

There are (at least) two big problems with such theories.

One, for each of these people we're not only told the age at which they died, but also the age at which they had the son who carried on the line. Adam had Seth at 130. If that really means 10 or 11, that's pretty young to have a son, and that seems a fairly typical age. And note that he had at least two sons before Seth. So he had at least three children by the time he was 11. That's almost as hard to believe as living to 900. Other patriarchs have similar ages for the birth of the son who carried on the line.

Two, as Narnian points out, after the flood the ages given rapidly drop to numbers similar to what we have now. So if Adam's 930 years really means 930 months or 77 years, then what about Joseph, who the same book of Genesis tells us lived to 110. Did he then only live to 110 months, or about 9 years? So he was sold into slavery in Egypt, his master's wife tried to seduce him, he was thrown in jail, eventually was realeased, became prime minister of a great empire, married and had two sons of his own. Quite a full life for someone who died at 9 years old!

Any theory that "year" is a mis-translation has to assume that it meant a period of about a month when giving the ages of Adam through Noah, the first 10 patriarchs; maybe 2 months for the next 3, Shem, Arphaxad, and Salah; 3 months for Peleg, Reu, and Sereg; etc, gradually building up to 10 or 11 months for Joseph; and then a true year for later generations. That's a remarkably flexible time unit.

As Narnian points out, the ages dropped dramatically beginning with the generation after the Flood. So the fair conclusion is that something changed after the Flood.

I used to adhere to the theory Narnian discusses: that the environment was dramatically changed after the Flood, the world was much less conducive to human life, and so life expectancies fell. But recently I read an argument that pokes a serious whole in that theory: the life expectancies dropped off over the course of several generations. If conditions after the Flood were so relatively harsh that people didn't live as long, then you'd expect that we would see Adam and all the pre-flood people living to 900, everyone born after the Flood living to 70 or so, and Noah maybe living to his age at the time of the Flood plus 70 years after. But that's not what we see. We see the ages winding down to 500, 400, 300, 200, 100, and then our present 70 or 80.

Thus I tend to think the better theory is that there was a genetic change after the Flood. This gets into theories about why we age, which is still not well understood. But an interesting theory today has to do with telomeres. Not to get into a heavy discussion of biology, but most of the cells in a human body can only reproduce a certain number of times, and so damaged cells can only be replaced a certain number of times. After that your body starts getting "used up". But some cells have a way to reproduce indefinitely. What if before the Flood all of our cells could reproduce indefinitely, but after the Flood there was a mutation that damaged this ability?

This is a great question. The Bible never provides a direct rationale for the seemingly long ages recorded in the Old Testament. It just states them as a matter of fact with no apology for them.

As we look more closely at the ages, though, we find some very interesting things. The ages fall quite dramatically at a very definite point in biblical history and decline quite rapidly to ranges that fit our current experience. This fall actually corresponds to the flood of Noah recorded in Genesis 6-8.

On a side note, it seems that Methuselah, Noah's grandfather, died in the year of the flood, while his father died some time before the flood. As Noah's was the righteous line, Methuselah is not thought to have died in the flood, but shortly before the flood. The name Methuselah actually means "his death shall bring judgment".

There is biblical evidence that this catastrophic event significantly changed the earth's climate in many ways. Indeed, we are told that prior to the flood, there was no rain, but that a mist arose and watered the earth instead.

Now no shrub of the field was yet in the earth, and no plant of the field had yet sprouted, for the Lord God had not sent rain upon the earth, and there was no man to cultivate the ground. 6 But a mist used to rise from the earth and water the whole surface of the ground. Genesis 2:5-6 NASB

The flood may have been the first instance of rain at all on the earth. So, if this is accurate, then the pre-flood climate may have been much much healthier than it was afterward. Plants would have grown much more quickly and been much more fruitful as well, and this would have been able to provide a plentiful food supply for larger animals.

If that is so, then that would explain many things that we find today:

• Wooly mammoths frozen with fresh grass still in their mouths
• tropical plants buried beneath many layers of ice in arctic regions
• skeletons of mankind that are enormous (giants mentioned in Genesis)
• Cave drawings and other depictions of dinosaurs, all created by mankind.

After the flood, as I mentioned before, we find lifespans plummeting to ranges within what we would expect today.

Prior to the flood, the wickedness of man had apparently risen to such a high degree that the only way God saw to remedy the situation was to basically start over with just one family.

Then the Lord saw that the wickedness of man was great on the earth, and that every intent of the thoughts of his heart was only evil continually. 6 The Lord was sorry that He had made man on the earth, and He was grieved in His heart. 7 The Lord said, “I will blot out man whom I have created from the face of the land, from man to animals to creeping things and to birds of the sky; for I am sorry that I have made them.” 8 But Noah found favor in the eyes of the Lord. Genesis 6:5-8

Some have suggested that God shortened the lifespans to prevent people from having time to descend to that level of depravity again, and this certainly is consistent with the biblical record.

Conclusion

So, the ages along with other information suggest a pre-flood climate that was extremely healthy and quite different from that post-flood climate. The ages drop significantly after the flood to levels similar to that of today.