忘记那些有关汉语时态的语法知识吧，都是错的。汉语没有时态，不要硬强加给汉语过去时、完成态之类的概念，一点好处都没有。 她在农村长大，是一个事实，是个前提条件，在这个前提之下，她了解农村才能成立。加“了”，不仅仅只是可以表达英语的“完成”，它还会强调长大的过程，因此“她在农村长大了”，会有“她在农村从是个小孩子一点点长在成人”的意思在里面，而这里是要强调她是在农村长大的，因此不需要这层含义，所以就不能加“了”，这和“她长大这件事已经完成了”没有关系。就好象中国说英语的时候经常觉得这件事完成了，所以就使用现在完成态或过去完成态，结果母者可能更多的只是使用一般过去时。道理是一样的。你心里总是想着，着了过，表示现在、过去、完成之类，而事实上汉语的着了过不是时态标志，和英语的时候不是一一对应关系。不放弃这种错误的观念，你的汉语很难学好。

To make it simple.. "了" is a modifier, it explicitly indicates a past event. Yes, your are right. But, "她在农村长大了" is not correct. We consider that it is a fact, and it is weird to emphasize the "past event". This is one of the case that we cannot translate English to Chinese word by word. "她在农村长大" is correct. Actually, we don't care about the tense in this case. English have it's unique rules on tenses, there is no equivalent thing in Chinese. Chinese grammar is actually easier and give you more "freedom". Your will found it is very easy once you learned the usage of "modifier" words.

了 generally have two structures. 1. Verb + 了 2. Sentence + 了 The first means the verb is finished. The second mean the sentence is finished. Anyway, 了 express "something has finished or is finished". If something can not be finished in short time, like living a life, 了 is not used in the sentence. If a sentence is not emphasized something is has finished or is finished, 了 is not used in the sentence.

This sentence is used to describe the fact that she grew up in the countryside therefore she is familiar with the life about the farmers. It is not trying to emphasize that she has already grown up. Hence the "了" is not necessary.

“她在农村长大，因此很了解农民的生活”。 In the sentence, “她在农村长大”，强调她生长的地方是“农村” It was a rural village where she grew up.( the emphasized point is “rural village”) 她在农村长大了，强调“长大了” She has grown up after many years of living in a rural village. ( the emphasized point is “has grown up”)