What is the ideal body weight and body mass index for a 20 year old female who is 5'6?

There is no 'average' height and weight for a 20-year-old woman. Weight is related to height and body composition rather than age. In addition, height tends to relate to nationality rather than age. For more information about the heights of women worldwide, and weight in relation to body composition, please see the page links, further down this page, listed under Related Questions.