Ishmael was somewhere between thirteen or fourteen years older than Isaac, and we find in chapter 17 that he was very tenderly loved by Abraham.
For a good while Abraham clung to the hope that Ishmael might be the promised seed. However God had other plans and the sending away of Hagar and Ishmael apparently was the only way God felt that the inheritance could be kept pure.
So Abraham sees that quite a number of years have gone by, by this time, maybe somewhere in the twenties.
Isaac means laughing and it is a reference to Abraham laughing in joy at the prospect of the possibility of Sarah giving birth to the promised son, and of course it is also a witness against them because of Sarah’s laugh of unbelief. And now here she is saying that everyone will laugh with me in joy and in amazement that I should give birth to a child at age 90.
What God did here was add circumcision as a requirement and as the sign that those bearing this mark had made the covenant at the same time with Abraham. (I believe that this term "selfsame day" appears something like fourteen times in the Bible.)
