In the vast majority of standardlised exams developed by ETS (Educational Tesing System) for example, TOEFL or TOEIC, to the best of my knowledge, candidates do not have to fulfill any requirements concerning their age. By inference, your sister (?) can have a chance to partake in these examinations.
However, TOEFL and TOEIC are much more prevalent to above-sixteen students. Your sister is still too young and, if she has not enrolled in any specialised training before, the assessment outcome may not be correct, which can lead to irrevocable disappointments. In that case, I do think you should spare a thought to possibly adverse effect which your sister may suffer in the long run. Draw an envision if your sister's result could not come up to her expectations. Would it be extended to her threshold ? Would she like to give it a shot ? There are myriad of questions which may influence your decision.
Have a good day!