Apparently the term l'as (the ace) was invented by French newspapers and given to early WWI French military aviator Adolphe Pégoud after he downed several enemy aircraft. The article I'm using goes on to say that once the American pilots joined the war in its late stages it looked unlikely any would achieve ten kills (the contemporary European standard to be called an ace) before German collapse, so they simply made the required number five, which then went on to become a global standard. It cites Raymond Toliver and Trevor Constable's 1965 book, Fighter Aces, as the source of this information.

The article can be found here: http://www.airspacemag.com/need-to-know/what-does-it-take-to-become-an-ace-35385936/