The Supreme Court has affirmed the constitutional right to self-representation in Faretta v California, but in Godinez v. Moran, the court injected the logically prior question of competence, rejecting a mixed-bag approach to competence. The court held that
when a defendant seeks to waive his right to counsel, a determination
that he is competent to stand trial is not enough; the waiver must
also be intelligent and voluntary before it can be accepted. While
States are free to adopt competency standards that are more elaborate
than the Dusky formulation, the Due Process Clause does not impose
The specific issue in this case is competence to plead guilty. In Indiana v. Edwards, the court stated that there is no absolute right to self-representation if one is competent to stand trial. It is held that
The Constitution does not forbid States from insisting upon
representation by counsel for those competent enough to stand trial
but who suffer from severe mental illness to the point where they are
not competent to conduct trial proceedings by themselves.
One conclusion to be drawn from this is that states may but are not required to insist on a specific competence to self-represent, so there can be multiple standards of competence. Second, the Edwards court put the issue more in the realm of questions of insanity, where the limit imposed on a technical minor would have to be a substantial finding of incompetence, and not just a statutory declaration that anyone below the age of 18 is ipso facto incompetent to legally defend themselves but they can be competent to stand trial.
In Martinez v. Court of Appeal of California, the court further held that nothing "requires a State to recognize a constitutional right to self-representation on direct appeal from a criminal conviction".
This study indicates that individual states have not eagerly cleared the path to minor self-representation, noting that "Many states permit waiver by a juvenile after cursory inquiry by the court", but "Others require that
the juvenile consult with a parent, lawyer or other adult". Nevertheless, "waiver of counsel is, almost without exception, connected to an 'admission,' or guilty plea", simply asserting that "Juveniles do not
represent themselves at trial", and what minors waive is the right to trial. There is a lack of relevant case law citations in this study, in that no ruling is cited where the court affirms that minors are automatically incompetent to represent themselves in a criminal trial. The specifics of the qualifier "almost without exception" would be very relevant to this question.