What will it matter to him if he notices that he is growing old? Has he any reason to envy the young people whom he sees, or wax nostalgic over his own lost youth? What reasons has he to envy a young person? For the possibilities that a young person has, the future which is in store for him? "No, thank you," he will think. "Instead of possibilities, I have realities in my past, not only the reality of work done and love loved, but of sufferings bravely suffered. These sufferings are even the things of which I am most proud, though these things are things that cannot inspire envy.
We're so arrogant, aren't we? So afraid of age, we do everything we can to prevent it. We don't realize what a privilege it is to grow old with someone. Someone who doesn't drive you to commit murder or doesn't humiliate you beyond repair.
Source: P.S. I Love You, screenplay by Richard LaGravenese & Steven Rogers based on the 2004 novel by Cecelia Ahern