The answer is: 90

Gen 17:17 / 21:5

In the heartwarming narrative of Sarah, we see God's incredible timing and the unfolding of hope. At the ripe age of 90, Sarah gave birth to Isaac, a significant moment in the Bible illustrating how life's possibilities are often unconfined by our expectations or age. This essential lesson resonates throughout the scriptures: God's plans often exceed human logic.

By having Isaac later in life, Sarah became a symbol of faith and resilience. She and Abraham remained steadfast in believing the promise bestowed upon them, often seen as directives for anyone aspiring to maintain hope even during difficult times. Interestingly, within ancient cultures, including that of Abraham's time, having children and maintaining family lineage was interconnected with identity and legacy, thus giving deeper meaning to Isaac's birth.

Additionally, Sarah's initial doubts and laughter at the notion of having a child (as seen in Genesis 18:12-15) demonstrates a very human reaction that many can relate to; she became the epitome of patience-producing faith. Her story unfolds in the broader context of a divine narrative where startling acknowledgments and satisfying resolutions often come post-trial.

To amplify our understanding of how unique and miraculous familial patterns can be, consider that Isaac's story unfolds a narrative that leads to other pivotal figures renowned in further Biblical writings, grounding a lineage through his own son, Jacob. This establishes not just a biological ancestry but a robust theological legacy as well, impacting three major faiths to this day.