Supple

As in:

Melvin was embarrassed by the accident with the truck and the horrible mess the nachos and the diet pepsi and the mentos created in the cab. It was as if a bag of tacos had exploded from inside a piñata, and it clearly put an end to the new truck smell. When George asked him how he could have missed the turn to get back to interstate, Melvin could only reply that whatever wisdom had come with age it had not improved his decision-making behind the wheel.

“Look,” he said. “My brain is not as supple as it used to be, George.”