Is there such a thing as “mature” Quenya and Sindarin?
No. As Tolkien himself wrote, his “process of invention” was undertaken to give “expression to my personal linguistic 'aesthetic' or taste and its fluctuations” (Letters p. 380, emphasis mine). At no time were Tolkien's languages fixed or finalized or free from even fundamental change or reconsideration; achieving such a state was explicitly not Tolkien's purpose. What is referred to by some as “mature” Quenya and “mature” Sindarin “of the Lord of the Rings era” are in fact artificially selected and dubiously homogenized sets of data spanning decades of “fluctuations”, which are nonetheless asserted to be essentially uniform in nature and conception. But in fact, most of what is claimed to be true of “mature” Quenya and “mature” Sindarin is actually silently asserted on the basis of evidence for the Qenya and Noldorin of The Etymologies. The “reasoning” underlying this representation is circular: Qenya and Noldorin of The Etymologies are more or less the same as Que
