The Persian language history has four period:

1- Avesta is the most anceint book that is written circa 3500 up to 2800 years ago (it is not certain) that was written in Din Dabire alphabet.

2-The Ancient Persian that was spoke during Achaemenes realm (2500 up to 2200 years ago) that was written in cuneiform.

3- The Middle or Pahlavi Persian that was the official language of Iran since about 2300 until 1400 years ago that was written in Aramaic alphabet.

4-The Dari Persian has been the official language since 1400 up to now. All great persian masterpieces has been witten in Dari Persian that was written in Arabic alphabet.

Fortunately, most of masterpieces of Pahlavi Persian has been translated to Dari like "Shahname" and "Kelile va Damne" and etc. The second good news is that my 3 years old daughter grammatically can understand them all (despite the meaning of the words :-)) ).

The Persian language has changed since that time.  But we steel understand it. We do not need to learn it like a foerign langauge. We almost understand Rumi's and other old poet's poems comepletely.

Salam. well our language has changed a lot but we still can understand some of his poems because we were taught some difficult words and phrases in school so we can understand it and if we have any truoble understanding it we try to find the meaning of that word in dictionary .and there are some classes in our country for learning his poems becasue some of them are hard to understand  but it`s not too much hard for us to understand it becasue we are native. but if you read soem literary texts and books you will be able to understand his poems and stories more that`s what we did at school to understand the old language.

But about Shahname i should tell you we just can understand the simplified version of it .