Two weeks ago, I joined in the celebration of my friend Dan’s birthday.

A birthday might not normally rate a column.  However, the carrot cake chosen for this occasion boasted three candles, marked one, zero, and zero.

Yes, my friend is now one hundred years old. I feel excited about his having reached the century mark, since he is the first of my close friends to do so.

As a person professionally interested in aging, I am fascinated by my friend’s longevity.  According to figures published by the Centenarian Study at Boston University, he is one among some 80,000 Americans aged 100 or more. Of these, about 85 percent are women, 15 percent men.

Only about 60 to 70 of Americans are over age 110. Researchers like to call them supercentenarians. No one of these, however, has come close to the well-documented record of the French lady Jeanne Calment who reached 122.

Though many of us attribute living long to the power of our genes, researchers such as Dr.Tom Perls of Boston University do not agree. Instead, they suggest that 70 to 80 percent of longevity comes from our environment.

Dan’s environment has been a remarkable one. He is a scholar, a university professor forced by the regulations of the era to retire 35 years ago. But since that time, he has continued a productive and distinguished career.

His work has earned him wide recognition as the first Americanist.  This designation applies to scholars focusing on American history and literature.  This field has brought many people, here and in other countries, to appreciate the growth and development of the intellectual life in the United States.

An important part in this enterprise was Dan’s part in the founding of the Library of America, which publishes outstanding scholarly editions of important American authors. The series includes more than 200 of this country’s greatest novelists, non-fiction writers, poets, and dramatists.

Dan and I often share a brown-bag lunch in his office, especially on holidays when virtually no one else inhabits his academic building. On those occasions we discuss the events and characters that have marked the past century. He has known many of the literary and political celebrities personally and I enjoy hearing his reflections about them.

In return, I sometimes regale Dan with my knowledge of ecclesiastical life, drawn from my first career. It helps that he and I share, in large part, the same worldview, especially in politics and culture.

Among other talents, he continues to be intellectually acute with powers of memory that impress me in their accuracy and their range.

Dan does sometimes complain about the limits of his memory. Most of the people I know who have reached 70 or 80 do the same. But he can summon up people, places, and things impressively.

He also has to bear physical disabilities.  Having to use a walker, for example, makes his daily trek to his office difficult. And he has had to endure various other problems that make daily life a challenge.

But he has persevered with amazing courage, inspiring me to deal better with my own comparatively minor problems.

How is it that so many others among my friends have died young but this special friend has flourished for a whole century?  This mystery persists in occupying my imagination, making me wonder how this can happen?

No one knows the answer, of course, but that does not mean it’s undeserving of attention.  I, for one will continue to ponder the why of it and ask what it all means.

I regard my friendship with Dan as a precious gift. Seeing him live so long I take as one of the unexpected benefits of my own later years.