a decrepit old age的用法和样例：
- A decrepit old man sat on a park bench.
- 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.
- Her widowhood condemns her to a lonely old age.
- We have saved some money against our old age.