title_body: What is the oldest known constructed language?
upvoted_answer: The oldest language whose creator set out to actually invent a language (as opposed to Pāṇini, who wanted to create a classical standard out of an already  extant language) was probably Lingua Ignota. The formerly brilliant Langmaker website had an article on it (now accessible through the Wayback Machine). It was made in the twelfth century, predating another early conlang, Balaibalan, which was made somewhen between the fourteenth and sixteenth centuries.