I'd say most girls should not be focusing on hard intervals in their early teens because almost invariably, the fast results that are seen in high school do not translate to successful longterm careers.
Girls get used to these heavy intervals and THEN their bodies change so that at least for a time period, they can't handle such training without getting hurt. The vicious cycle includes getting fatter naturally, then getting hurt out of trying to live up to past performances with a very different body, and getting still fatter while getting injured. This frustrating process works in cycles.
If I had a teenaged daughter that wanted a future in running I'd be very reserved about getting her too race/ interval hungry until after high school.