a decrepit old man的用法和样例：
- A decrepit old man sat on a park bench.
- The decrepit old man led a poor life.
- Pale and limp as a rag, the prince lay on his bed waiting to die, when in walked his old nurse, a decrepit old soul now who had nursed him as a baby and who still worked for him.
- That old man is a fount of wisdom.
- The old man is scourged by a guilty conscience.