"I fear the end of the year," a friend recently told me. "It makes me realize that I am getting old."
The great psychologist William James once remarked that most people become conservative and outdated by the age of 25. He was right. Many people are satisfied with their jobs by 25. They have accumulated biases they call "principles," closing their minds to all new ideas; they have simply stopped growing.
Once a person stops growing—regardless of their age—they begin to age. Conversely, true greats never grow old.
Goethe was 83 when he passed away, yet he was still working on "Faust" in his final years; Gladstone began learning a foreign language at the age of 70; the French astronomer Laplace was still working when he died at 78. Before his death, he exclaimed, "What we know is so little, and what we do not know is so much."
From these examples, you already have the true answer to the question: "When does a person grow old?"
Laplace was still young when he died at 78. He remained unsatisfied and felt there was still so much more to learn.
As long as a person maintains this mindset, as long as they can look back on the past year and say "I am growing," they remain young.
The moment a person stops growing, the moment they say "I know enough," their youth is over. It makes no difference whether they die at 25 or 75. On that day, they began to age.