Man is like a car.

As it get older, the differential starts slipping, and the u-joints get worn, causing the drive shaft to go bad.
The transmission won't go into high gear and sometimes has difficulty getting out of low. The cylinders get worn and lose compression, making it hard to climb the slightest incline.
When it is climbing, the tappets clatter and ping to the point where one wonders if the old bus will make it to the top. The carburetter gets fouled with pollutants and other matter, making it hard to get started in the morning. It is hard to keep the radiator filled because of the leaking hose.
The thermostat goes out, making it difficult to reach the operating temperature. The headlights grow dim, and the battery need constant recharging.
But if the body looks good, we can keep it washed and polished, giving the impression it can compete with newer models and make one more trip down the primrose lane before the head gasket blows.

Gentlemen, start your engines.