In the Old Testament, when people lived to be hundreds of years old -- did they really live years as we know it or did they count seasons as years? How could anyone have lived to be 400 years old?
The word used for "years" did not change meaning in the Old Testament. Given that Moses is ascribed to be the writer or editor of Genesis, a year in his language would be the same as a year in the things he wrote about.
Even if you claim that a year in the early part of Genesis meant a season, it wouldn't solve your problem of long ages. For example, Mahalaleel lived to be 895 years old. Even dividing this by four would give you 224, still a very old age. But such doesn't work. Shelah lived 433 years (108 if we divide by 4), but he had a son at the age of 30, which would mean that if we divide 30 by four that you are claiming he had a son at the age of 7 and a half! No, it is obvious that a year means a year.
One type of logical fallacy is to measure everything against only the things that you know. For instance, people in the 1700s would (and did for that matter) argue that it is impossible for men to fly.
When you look at the ages recorded in the Bible, the people who lived before the Flood lived incredibly long lives compared to our standards. But after the Flood, the number of years a person lives quickly drops from generation to generation. By the time you get to Jacob, we find that Pharaoh is amazed at his great age of 130 years. "And Jacob said to Pharaoh, "The days of the years of my pilgrimage are one hundred and thirty years; few and evil have been the days of the years of my life, and they have not attained to the days of the years of the life of my fathers in the days of their pilgrimage."" (Genesis 47:9). By the time we get to King David, we find that 70 to 80 was typical. "The days of our lives are seventy years; and if by reason of strength they are eighty years, yet their boast is only labor and sorrow; for it is soon cut off, and we fly away" (Psalm 90:10).
Obviously something changed with the Flood, though no one knows what it is. It could be a genetic change, such as a mutation in our genes, but given the wide-spread nature of the change, it is more likely an environmental change that prevents us from living as long as we once did. It is interesting to note that our ages are slightly increasing once again. The number of people living past 100 is increasing and we frequently find people living to be 110 to 120 years.
One intriguing idea that has been recently brought up is that even after our bodies reach maturity, certain parts of our heads continue to grow, though at a very slow rate. In fact, a very old person would begin to look like the fossils we label Neanderthal. Thus, there is speculation that the Neanderthal fossils are the remains of people who had lived incredibly long lives. Whether this is true would be very difficult to prove