And there you have the real answer to the question, "When is a man old?"
Laplace at 78 died young. He was still unsatisfied, still sure that he had a lot to learn.
As long as a man can keep himself in that attitude of mind, as long as he can look back on every year and say,"I grew," he is still young.
The minute he ceases to grow, the minute he says to himself, "I know all that I need to know,"--that day youth stops. He may be twenty-five or seventy-five, it makes no difference. On that day he begins to be old.