In my collection is a set of vintage Valentine's Day cards, with most addressed to someone named Martha Stewart (and coincidentally the modern day Martha Stewart has some online templates for designing Pop-Up Valentine cards). I believe most of these shown are from the 1920s. I'm not sure if its just a characteristic of my lot of cards, or if it was a rarity to see adults portrayed in early 1900s valentine card designs. Almost all of the cards I've seen from the era depict little children, or illustrate Victorian style florals, and or incorporate the heart shape.
Mass production of American Valentine's Day cards began in the 1940s, with much credit to the designer and artist, Esther Howland. Her name was Esther Allen Howland (1828-1904), and today she is often called the Mother of the American Valentine's Day card. Her alliances with two other early valentine makers, Jotham Taft (1816-1910) and George C. Whitney (1842-1915), would build the company she started in her home into an economic powerhouse (excerpt from article by David Kubicek). The Lilly Library provides an informative history on Victorian style cards online, where you can also send a Valentine's themed e-card (thanks Brian).