Why was God's promise to Abraham still valid despite his and Sarah's old age?
The promise remained valid because God said He would do it and would not let it fail; its certainty did not depend on human ability. Though Abraham was nearing 100 and Sarah nearing 90, God promised them a son named Isaac, who would be a son of promise and a miracle.
Les explains that what God promises, He is not going to let fail. So in spite of Abraham nearing 100 and Sarah nearing 90, the promise is valid because God said He was going to do it. God told Abraham He would give him a son, and he was to call his name Isaac. In Genesis 17, God also dealt with Ishmael, promising to make him fruitful, multiply him exceedingly, and make him a great nation, but God said His covenant would be established with Isaac, whom Sarah would bear at the set time in the next year. Isaac was therefore a son of promise and also a son of the miracle-working power of God, because both Abraham and Sarah were way beyond normal childbearing.