Elie Metchnikoff

The appearance of aged persons is too well known to make detailed description necessary. The skin of the face is dry and wrinkled and generally pale. The hairs on the head and the body are white. The back is bent, and the gait is slow and laborious, whilst the memory is weak. Such are the most familiar traits of old age.

By irreducibly complex I mean a single system composed of several well-matched, interacting parts that contribute to the basic function, wherein the removal of any one of the parts causes the system to effectively cease functioning.

You can open up a centenarian's brain, and you'll see some areas that look like that of a 50-year-old or of a 110-year-old. You can have variation in the basic process of aging, called senescence, in different parts of the same body.