How tf does one user turn someone else’s account into a junior account?

The other day, one of my friends (we’ll call him A) told one user (B) to make his other friend (C) end up with his account being turned into a junior account even though it doesn’t need to be. Well B actually did it somehow and C’s account is now a junior account. C made a new account because he is most definitely within the age range to have a full account. There’s more drama to it than that but this is the basic version of the story. How tf is this possible at all?