FOR Daniel Ryan, turning 40 and 50 came and went. But sometime around his 61st birthday last July, he said he had an “old man crisis.”

He did some simple math, and quickly concluded that living another 20 years would be considered a “full life.” Even if he remained healthy for another 15 years, he figured he might only be able to maintain his active lifestyle of skiing, hiking and mountain biking for another decade. “Looking at the rest of my life this way hit me like a ton of bricks,” Mr. Ryan said.

Image: James Best Jr./The New York Times