The horror of underage smoking veils a horror of teen and preteen sexuality, and one of the biggest pleasant empty dreams being pushed these days by Madison Avenue is that a child is innocent until his or her eighteenth birthday. The truth is that without firm parental guidance teenagers make all sorts of irrevocable decisions before they're old enough to appreciate the consequences--they drop out of school, they get pregnant, they major in sociology. What they want most of all is to sample the pleasures of adulthood, like sex or booze or cigarettes.