There’s a spectrum between conventionalised and artificial, and Sanskrit is somewhere along that spectrum. Specialists other than myself can answer better than I as to how artificial Sanskrit is.
We have no idea how old the Aboriginal initiate language Damin is, and therefore whether it is older than Sanskrit or not. It is clearly much further along the artificial axis than Sanskrit is, although it is still based on the Aboriginal vernacular language Lardil. (Read about it: it really is an astonishing language.)