As can be seen through this blog thread series, illustrations tended to originally focus on adults, with screen paintings, illustrations for "Tales of Genji" and the "manga"-style ukiyo-e woodblock prints. When we get to the second half of the 1800's, manga as "caricature" and social and political commentary is very specifically targeted towards adults in the magazines and newspapers. The first few attempts to create manga for children failed. It's not until 1908 and the start of Shojo no Tomo magazinethat we start seeing artwork aimed at children taking form and being successful. Takei Takeo, Takehisa Yumeji, Katsudi Matsumoto and Suihou Tagawa then laid the ground for manga as entertainment for children in the 20's and 30's.
Tezuka first appears in 1946, and makes a splash with his "Shin Takarajima" in 1947. From here, he establishes himself as a children's cartoonist in Shojo no Tomo, Shonen Kurabu and Manga Shonen. The Tokiwa Manor Gang follow, with Shotaro Ishinomori's "Angel, Second Class" appearing in Manga Shonen in 1952. The constant influx of artists through the 50's in children's magazines now cements the public impression that "manga is for kids". During the early 1960's there was a pressure building up for the release of a new kind of publication that would allow for "realistic" stories aimed more towards adults. That is, while Tezuka had broken new ground by writing story-driven comics, the ground only covered a limited age range. In 1964, Katsuichi Nagai decides to rebel against "manga is only for kids" by adopting the phrase "gekiga" to represent "graphic novels as serious art", and launching his own magazine, named Garo. (Manga artist Yoshihiro Tatsumi is credited with coining gekiga in 1957. Tatsumi's own latest work to appear in the U.S. is his 800+ page autobiography "A Drifting Life", which hit the shelves back around April, 2009.)
One reason for mentioning Garo in this thread is that the western press has been picking up on the U.S. releases of several translated titles from Garo, such as Susumu Katsumata's "Red Snow". Many anime fans may be familiar with the first serialized strip to run in Garo - Sanpei Shirato's "Kamui". Garo can probably be described as the predecessor to western hard-core graphics magazines like Art Spiegelman's RAW (1980-1991). According to one source, the gekiga movement then worked to influence Tezuka, who afterwards drew a number of more adult titles, leading up to "MW" (1976-78), while also creating the copycat magazine COM. Garo reached its peak in the 70's, then went into a slow death through the 80's and 90's. It was bought out by a game company, which tried to revive it, but the magazine finally folded around 2002.
Just as in the U.S., where cartoons started out aimed at adults, then got relegated to "childish", then recovered under "animation for adults" via "Fritz the Cat", "Heavy Metal" and "Metalocalypse", the same occurred for "manga". There are hundreds of titles aimed at adults now, with themes ranging from cooking, golf, pachinko, mahjong and politics, but they're all included under the category of comics or manga. The only time I see gekiga pop up now is when someone wants to talk about why Garo was different from other contemporary magazines.