Certainly the discussion around the Council of Canadian Academies table was that “decisional capacity” is a much better term than “mature minors” because “mature minors”, as a legal term, is understood by lawyers and judges, but is not so clearly understood by members of the public. “Decisional capacity” better describes what it is that you're actually looking for.
I understand from that discussion that among medical professionals they do not use “mature minor” usually. What they'll refer to is individuals with “decisional capacity”, and that's true whether the individual is 14 or 94. The same issue applies.