I want to know the answer to the following question:
What language shows a level of maturity corresponding to Old Avestan?
All the background I'm provided with is that "On the other hand, Younger Avestan is at about the same linguistic stage as Old Persian, but by virtue of its use as a sacred language retained its "old" characteristics long after the Old Iranian languages had yielded to their Middle Iranian stage.". Is that enough to answer the question?
Answer: no