They may be a bit shaky in the legs, but there is nothing wrong with their mental and creative faculties! Elliott Carter, the centenarian, has composed works as intricate as ever over the last few years, and Charles Rosen at 81 is still playing Carter's complex pieces here at the Carterfest at Tanglewood this week, including the Double Concerto for Harpsichord and Piano (with Ursula Oppens on the harpsichord) which he helped premiere in 1961. And they are both accessible: Carter sits mostly in the audience, and Rosen is not averse to having a casual conversation with a concertgoer.
I found two excellent related articles in the NYT archive, one by Mr. Rosen himself: