Tierney's main point seems to be that older workers should be willing to take jobs they now consider beneath them. He calls this the "Adams Principle," in honor of John Quincy Adams, who served in Congress after he was President:
Adams started his new career at age 63, just about when the typical American man now retires. He wasn't especially spry, once calling his body "a weak, frail, decayed tenement battered by the winds and broken in on by the storm." Yet he stayed on the job until his death at age 80.
He accomplished so much in those years that he is remembered as a better congressman than president. You could call him an inverse example of the Peter Principle, someone who succeeded by being demoted below his level of incompetence.
But I prefer to draw a different lesson. Call it the Adams Principle for employees and employers: if the president can flourish after a demotion, so can anyone else.