When the fire slackens, life declines; when it goes out, we are dead.
 Oliver Wendell Holmes Sr., The Autocrat of the Breakfast-Table (1858). copy citation

Context

“The human body is a furnace which keeps in blast three-score years and ten, more or less. It burns about three hundred pounds of carbon a year, (besides other fuel,) when in fair working order, according to a great chemist’s estimate. When the fire slackens, life declines; when it goes out, we are dead. It has been shown by some noted French experimenters, that the amount of combustion increases up to about the thirtieth year, remains stationary to about forty-five, and then diminishes. This last is the point where old age starts from.” source