What does Genesis 7:6 mean?

In Genesis 5:32, we were told that Noah was 500 years old when he fathered his three sons. Then in chapter 6, God told Noah to build the ark. Now we learn that Noah was 600 when the flood came. That suggests that Noah might have had as long as a hundred years to build the ark and prepare for the flood, though some scholars suggest that God's revelation to Noah came much closer to the flood itself. In any case, God certainly knew how much time it would take Noah to complete his work, and that this work would be completed just in time (Genesis 7:13).

While the idea of a 500-year-old man building a box the size of an apartment building seems outlandish, the context of the story is crucial. In this era, during the time from the Garden of Eden to the Flood, people had been living to be nearly 1,000 years old. A man of 600 would be in his active years, the equivalent of middle-age. Human lifespans will begin to decrease dramatically after the flood (Genesis 6:3), but for now, Noah is not an "old man" in this context. And, with so much time to work, and at least his sons to help, there's no reason to think he could not have completed the task he was assigned.