“Mature minor” means somebody who has decisional capacity with respect to the specific decision they're being asked to make. They need to be in a position to be able to understand the risks and benefits of the decision they are being asked to make, fully understand the implications of the decision, and be able to make an informed decision, understanding all of the implications of that decision. That is what I would understand to be the mature minor.
“Decisional capacity” is the other term that people prefer to “mature minor”. Thank you.