Hello @ confuz ed, Thank you for reaching out with your question. The first step would be to update the parental controls. This will allow the 15 yr old to use the Mobile App. Here is what you must do first.
Now if you have already done that or if you have now completed that, they should be able to download the app on their phone. They do have limited things that they can do but once they get on the app they can explore what they have or don't have access to do until they are 18.
Hope this helps. ~ Suzy