It is important to remember that our forefathers never accepted age as an excuse.
Perhaps that is one of the reasons that Abraham was circumcised when he was ninety-nine years old – so that no one would be able to claim advanced age as an excuse to avoid starting on a new path.
That person would simply look at Abraham, who at ninety-nine made a new start from the same place where an eight-day-old baby begins.
Even someone who has seemingly reached the latter part of his life, generally a time of decline, can still grow.
After all, all of Parashat Vayera transpires at a time when Abraham, one would think, should have been sitting calmly in his rocking chair – he was about one hundred years old, yet it was at this stage that he experienced many of the momentous and dramatic events of his life.
–Rabbi Adin Steinsaltz
From Talks on the Parasha