with和for当因为讲的区别

下面这句话为什么用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是介词；而for表示“原因”时,通常用作连词,后接从句.例如：We can't go for it is raining.我们不能走,因为正在下雨.