with和for當因為講的區別

下面這句話為什麼用with不用for:The reason why she had not stopped at the red light was most probably that her eyes had become weak (with)old age.分析：with是介詞；而for表示“原因”時,通常用作連詞,後接從句.例如：We can't go for it is raining.我們不能走,因為正在下雨.