TSK · Genesis 21:5

Abram was eighty-six years old when Hagar bore Ishmael to Abram.

Then Abraham fell on his face, and laughed, and said in his heart, "Will a child be born to him who is one hundred years old? Will Sarah, who is ninety years old, give birth?"

After that, his brother came out, and his hand had hold on Esau's heel. He was named Jacob. Isaac was sixty years old when she bore them.

Without being weakened in faith, he didn't consider his own body, already having been worn out, (he being about a hundred years old), and the deadness of Sarah's womb.