This is, of course, also a mainstay of American culture, only there the sex objects are usually called teens. Of course, the sets of female teenagers and schoolgirls are
nearly identical. The difference is that the school uniforms that are mandatory in all Japanese middle and high schools make the "schoolgirl" aspect more obvious than the age aspect. Girls aged between about 16 and 20 are considered highly desirable almost universally, probably because of evolutionary reasons (they are the most fertile). And in Japan, the vast majority of girls in this age group whom you see on the street are wearing school uniforms.