If we understand the Bible literally (or a better term- conversationally, in the same way you know a person talking to is being literal or being figurative) than that means ages are factual events. This isn't to say there is no figurative language in the Bible or that all numbers are literal representation of facts (most of them are, but this is a fairly complex and debated issue among st scholars about particular numbers).

It is a little hard for us to believe that people lived upward nine-hundred years at the time. The Book of Genesis has the characteristics of historical narrative literature, which was meant to represent historical detail (though the primary purpose of Bible history is not create a conclusive record of every event). 

Genealogies in Genesis like the Book of the Generations of Adam (Genesis 5) assign various ages in the hundreds of years for many people. Similarly, the flood narrative describes 120 year countdown to the Flood (and a reduction of lifespan to about 120 years^[1] though in today's world it is even less) and that Noah lived 350 years after the Great Flood^[2]. There is a definte literal implications here. The problem with not taking the ages as they are is there isn't anything they could represent. If you look through the genealogies you will see a wide variety of numbers for years. There isn't a correlation or anything that they could be symbols of. That's a good indication that the authors of the Bible believed them to be real years or thought people would take them seriously. 

Here's a good article and another one.

Basically, the Bible has a lot of unbelievable things in it. Everything from how long people lived to a man being God incarnate that performed miracles are unbelievable, especially for us in our age of rationalism and fear of the supernatural. Genesis 6 makes it clear that, because of man's wickedness, he reduced the lifespan so that they would no longer live all of those years. 

It's not that these crazy things are just thrown in the Bible and we are expected to blindly believe. We can investigate and test the things of the Scripture and if its the Word of God it will withstand the test. There are some things in the Bible and in other fields like science that can't be proven. There is an element of faith, but its not unreasonable faith. 

People used to live that long, much like many humans used to exceed nine feet tall, now its extremely rare for that to happen. 

1. ↑ Gen 6:6 (Link)
2. ↑ Gen 9:28 (Link)