What does Genesis 21:8 mean?

Not only was the miracle child Isaac born, he survived being weaned. Compared to modern times, this era would have had extremely high infant mortality rates. A substantial number of infants did not survive to become independent adults. A child who survived the helplessness of their early years, until they were no longer directly dependent on their mother for food, took a step towards surviving to adulthood. Women of this time may have breastfed their children as late as two or three years old, so Isaac was likely a toddler on the day of this great feast thrown to celebrate his life.

As earlier verses indicated, there is no doubt that Isaac's birth was entirely miraculous. His parents are decades beyond the normal age to bear children. His mother had previously been unable to conceive. Despite the doubts of Abraham and Sarah themselves (Genesis 17:17; Genesis 18:12), God's promises came true in the form of this long-awaited child.