In the UK we say that the minimum age is 11 or Year 6 - and only as a preparation for high school. I'd ask other countries to consider how it could best be used to get young people ready for high school and would say 6th or 7th grade as the lower age range. The key thing is that we want to protect the "teen" brand for the sake of our older learners. I wouldn't want any 15 year old finding out that their 9 year old sister was getting the same help. Our learners have had enough humiliation and TRT is too mature for younger children