Today, when so many women in the Western world refuse to nurse their babies, or nurse them for only six to nine months, a five-year period may seem inconceivably long. But Dr. D. B. Jelliffe reports that in many parts of the world children are not weaned until they are one and a half to two years old, and in Arabia it is customary for a mother to nurse her young anywhere from 13 to 32 months. Medically speaking, nursing, or lactation, may normally be continued until the next pregnancy is some few months advanced.​—Infant Nutrition in the Subtropics and Tropics, Geneva, 1968, p. 38.

In the Middle Ages in Europe the average age for weaning was two years, and in the time of the Maccabees (first and second century B.C.E.) women nursed their sons for three years. (2 Maccabees 7:27) Four thousand years ago when people lived an unrushed life, and there was not the present-day pressure or necessity to telescope so much into the shortened life span, it is easy to understand why Sarah could have nursed Isaac for five years. Besides, he was Sarah’s only child after many years of barrenness.

This means that Isaac was about five years old when weaned, having been born in 1918 B.C.E.