It's not age. Some great movies that bear close examination have been made within the last decade. So what is it that makes a film "classic." I'd have to answer with a cliche: "It's a combination of elements!" Since I work directly with cameras I want to focus on cinematography.
Let's agree on one thing, films are a work of art and therefore open to a huge amount of interpretation depending on the mind set of the viewer. But there are certain objective measures one can apply to a film to judge it against other films. Since I am the director of photography here at Joyco, I like to compare how films are photographed and contrast the techniques of modern versus old films.
I think the term "classic" can legitimately be applied to photographic techniques used on older films, simply because they have been around longer and the styles used by cinematographers of the 30s, 40s and 50s have been studied and studied and repeated by others. Also, many of those films were shot during the Golden Age of Hollywood, when cinematographers were often under contract to a studio and were assigned to a film because they had refined a style of photography the studio liked for musicals or another style for romance films. In other words, these guys were very practiced at their art and had a vast stable of equipment they could call upon to achieve a desired look for a film that pleased both studio execs and audiences alike.
But I still haven't answered my initial question: "What makes a film's cinematography classic?" In my mind, classic cinematography is memorable. When the images are burned into your consciousness, then I think, you truly have "classic" cinematography.
On the Internet today, you see lots of lists: ten foods you should never eat; 15 very stupid celebrities and so on. But you don't seen many lists extolling the photographic virtues of films, so I decided to start my own. Lets start with 1941's, Citizen Kane, an amazing film combining a visionary director with a remarkable cinematographer, Gregg Tolland, to produce a truly ground breaking B&W film. Next I would have to single out all six Lord of the Rings films starting in 2001, photographed by Andrew Lesnie. Each shot is so carefully composed it is truly a work of art. Here again the combination of a brilliant director and an artistic cinematographer produce a series of films that will be viewed for years to come and held up as examples of "classic" film making.
Obviously there are many, many more films I could site as "classic cinematography" examples. But try this: watch a movie on TV with the sound turned off. Pay extreme attention to the picture: the angle of light for each scene; how the camera moves; the composition of the elements within the frame. Do this and I guarantee you will find some classic movies yourself that will be visually memorable.