In the "general Westerosi view," girls may well be wed before their first flowerings, for political reasons, but it would be considered perverse to bed them. And such early weddings, even without sexual intercourse, remain rare. Generally weddings are postponed until the bride has passed from girlhood to maidenhood. 
For girls, the first flowering is also very significant... and in older traditions, a girl who has flowered is a woman, fit for both wedding and bedding. A girl who has flowered, but not yet attained her sixteenth name day, is in a somewhat ambigious position: part child, part woman. A "maid," in other words. Fertile but innocent, beloved of the singers.