What does Genesis 11:12 mean?

The line from Noah to Abraham follows through Shem to Arpachshad and now to Shelah. This is a noteworthy point in Genesis due to the age of Arpachshad when he fathers Shelah: "only" 35 years old. This is a drastic reduction in the age of having children when compared to the pre-flood era (Genesis 5). Of course, the genealogies listed prior to the flood don't demand those children were the first-born, so those patriarchs might well have had other, earlier children. And yet, it's significant that the ages associated with children are decreasing, as are the lifespans.

Whether as a result of the world being damaged by the flood (Genesis 7:22–24), or by God's direct intervention (Genesis 6:3), humanity's lifespans will taper off very quickly in the generations following Noah. To multiply and fill the earth, people had to start having children at much younger ages.