Genesis 17:1 records God commanding Abraham to walk blamelessly at age 99; Abraham was 100 when Isaac was born, as stated here — confirming God's promise.

In Genesis 17:17, Abraham laughed in disbelief at fathering a child at 100. Here that exact age marks the fulfillment of the promise he doubted.

Romans 4:19 references Abraham being about 100 to highlight his body being 'as good as dead' — the very age confirmed here at Isaac's birth.