When your youthfulness is not significant any more...

The picture is for the title. Actually, I borrowed it from a friend's article about his life path. (He's an econometrician. That's why he used the word "significant".) I was moved so much by the article, maybe just because of the title. It reminded me of a comment from an old professor during a talk in last weekend: "You are not young enough to pursue a topic so ambitious". And I replied, "Yes, I'm rational enough to understand that. But I can't help myself".