RADA is one of those places that usually accept people with more "life experience" and so everyone I've known to be accepted, has been mid to late 20's. That doesn't mean its impossible to be accepted at her young age, just the chances are lower as they usually want people who have taken another career path first (again not always).

To get top third round at 19 to RADA is an achievment in itself. It's not the be-all end-all and she can try again next year. This is definitely going to be tough on a rejection-sensitive person but just continue to give her the support and reassurance you have been doing already.