Aging as Losing the Wheel
In a car, the steering, brakes, and accelerator gradually fail. At first, you can still swerve around a pothole, but each year the maneuver takes more effort. At some point, even a perfect road becomes impassable — the car no longer obeys the driver.
That's exactly how the authors describe aging: not as the accumulation of rust or wear and tear, but as the loss of "controllability" of the body.