Next month, Jean-Pierre Serre will deliver a series of talks in the Distinghuised Lecture Series at the Fields Institute on several new topics. At 86. [edit: the 3 videos have now appeared: great talks!]
Looking for mathematicians who were born before him in the MacTutor website, it seems that Freeman Dyson is still active at 89, and Louis Niremberg, at 88, too.
Very impressive, and certainly as inspiring as centenarians that run, or cycle, or teach fencing.
Perhaps the reader will, one day, do even better, who knows?