句子主干：
It is difficult.for the average reader.to imagine a time.
（即：It is + adj.+ for sb.+ to do sth.的结构）
to the point of 到...的阶段（程度,地步）
to the point of impossibility 程度状语,修饰difficult,指困难到了什么程度.
under that age of forty 介词短语做定语,修饰the average reader,the average reader under the age of forty to 四十岁以下的普通读者
when high-quality arts criticism could be found in most big-city newspapers 定语从句,修饰a time.
翻译：对于四十岁以下的普通读者而言,想象曾经有一段时间多数大城市报纸上都可以找到高质量的艺术评论是极其困难的.