People like Methusela being 900 is okay, because his age seems to be a central factor in his importance, but it seems as though every other major person in the Bible lived to be over 100. That's outrageous even now. Is there some cultural significance to the ages I'm missing?
Okay, a few points.
There are other ancient (even earlier) parallels of this phenomenon. For example, a Sumerian king list lists figures living over tens of thousands of years.
Often the numbers have a theological or symbolic purpose for the authors. For example, Enoch lived 365 years. This could perhaps have been some type of connection to a lunar calendar. Do note that he was said to have "walked with God", this shows the symbolic tendencies of the authors of Genesis. Walking with God meant that this person did not experience death, he proceeded to live in the divine realm. Later Jewish writers use Enoch for all kinds of purposes.
Lifespans diminished as Genesis continued. I don't want to get too technical, but the people who preserved and collected the biblical traditions also wanted to express that God was increasingly becoming less pleased with the Israelites, coinciding with this was the diminishing life spans. So they preserved a trend of as God became displeased, he also decreased their lifespan. You have to remember, the ancient Jews viewed the hardships they faced as divine punishment. When their kingdoms were destroyed it was because of idolatry. Similarly, when they succeeded it was also because of the deity.