D'Artagnan was supposed to be 20 at the beginning of the film, although Gene Kelly was 35. Curiously however Dumas described D'Artagnan as being 39 or 40 in January 1648 at the beginning of the sequel "Twenty Years After".
Neither France nor England had prime ministers, in the modern sense, in the 17th century. However, Richelieu, was Louis XIII's First Minister of State. He was the King's senior Minister and conducted national affairs in his absence. The Duke of Buckingham had no similar position, but he held important Ministerial posts under James I of England and Charles I of England. He was Charles' favorite most trusted adviser for many years. Calling them First Ministers or Prime Ministers is easy shorthand for movies because of their influence.
Gene Kelly was hoping his performance would convince MGM to let him do a musical version of Cyrano De Bergerac. Regardless of his energetic performance, the studio brass wouldn't go for a musical Cyrano even though Kelly pestered them for years about it.
Fearing pressure from church groups, MGM had the script refer to Richelieu as Prime Minister rather than Cardinal and almost all traces of him being a cardinal or a man of the church at all have been removed, even though other versions of this story kept Richelieu explicitly a cardinal without any repercussions.
A December 1947 Hollywood Reporter news item noted that director George Sidney planned to shoot his own version of the film with a 16mm camera, alongside the studio's 35mm cameras, but no further information regarding a 16mm version has been found.
This film's initial telecast in Los Angeles took place Friday 3 January 1958 on KTTV (Channel 11); it was not aired in Phladelphia until 1 November 1958 on WFIL (Channel 6), followed by New York City 5 December 1958 on WCBS (Channel 2); San Francisco televiewers finally got their first look at it 27 August 1960 on KGO (Channel 7). At this time, color broadcasting was in its infancy, limited to only a small number of high rated programs, primarily on NBC and NBC affiliated stations, so these film showings were all still in B&W. Viewers were not offered the opportunity to see these films in their original Technicolor until several years later.