Here are some functions that tend to decline with age, but not always so, according to Reubin Andres of the National Institute on Aging:
Lungs: "The ability to move air in and out of lungs with vigor is considered a youthful characteristic." Non-smokers at 70 tend to have physiologically younger lung capacity than smokers at the chronological age of 45 or 50. (President Reagan stopped smoking years ago and took up jelly beans.)
Kidneys: "Kidney funtion declines markedly with age. From early adulthood to about 80, the average person loses almost half his kidney capacity."
Heart: Age can produce changes in the vigor of heart contractions.
Ability to dispose of glucose: "This also declines with age. We were probably overdiagnosing diabetics in older people."
Ability to metabolize alcohol: "The ability to dispose of alcohol does not change, but the effects are markedly different. Older people do not feel the effects of alcohol subjectively as much, but their performance decreases to a marked extent." A function that doesn't change very much, says Dr. Andres, is nerve-conduction velocity. And although memory does seem to become shorter, "one thing that does improve is wisdom. And we hope that might have something to do with our president."