Since I responded in your thread, I’ll link it here to provide closure.
And as for the OP:
Because I was doing DS13 a week before his 13th Bday (Disney’s minimum age to have your own account), I had them link him to me (his only trip with a MB was with his aunt), and had to wait to get him an account until he turned 13. Had to get the linking done then in order to set up his tickets and FPs.
It is therefore my understanding that you can have an “underage” friend that is linked to multiple users accounts, but that it can only be done by a “privileged user” at the helpdesk. Call them for assistance.