What does Genesis 18:11 mean?

Abraham and Sarah were around 100 and 90 years old, respectively. Given the long lifespans of early mankind (Genesis 5), it stands to reason that people of Abraham's era continued to have children later than they do today. At the same time, the text spells out that they were definitely considered old. At this point in human history, 90-year-old people were not thought of as young, by any stretch of the imagination.

In addition, not only has Sarah been barren her entire life, she has also passed menopause. As the ESV puts it, "the way of women had ceased to be" with her. By every measure, then, it would be physically impossible for Sarah to conceive and bear a child. Only God could cause such a thing to happen at this point in her life.

That, of course, is exactly what is about to happen. Sarah is listening from inside the tent as Abraham converses with three strangers (Genesis 18:9-10). These men are actually God, in human form, along with two angels. Their prophecy to Abraham echoed the same guarantee given to Abraham alone in an earlier encounter: Sarah will become pregnant!