We have several questions that fall in line with this topic: namely this question about Cardinal Mezzofanti, and this more recent question about Celine Dion. Are these questions on-topic? How can one determine if an individual is well-known enough to ask about here?
I consider the current two examples (Mezzofanti, Celine Dion) as on topic. I doubt that we can establish a degree of "fame" that can serve as a threshold. Mezzofanti is fairly well-known in the polyglot community; there is at least one site that was named after him. Céline Dion is definitely famous, but I don't know whether we'll find anything about her learning process. However, "there is no information on x is not a valid close reason", as far as I know.