§1. Minors and those who lack the use of reason can stand trial only through their parents, guardians, or curators, without prejudice to the prescript of §3.
§2. If the judge thinks that the rights of minors are in conflict with the rights of the parents, guardians, or curators or that the latter cannot adequately protect the rights of the former, then the minors are to stand trial through a guardian or curator appointed by the judge.
§3. Nevertheless, in spiritual cases and those connected with spiritual matters, if the minors have attained the use of reason, they can petition and respond without the consent of their parents or guardian. They can do so personally if they have completed their fourteenth year of age; otherwise, they do so through the curator appointed by the judge.
§4. Those deprived of the administration of goods and those of diminished mental capacity can stand trial personally only to answer for their own delicts or at the order of the judge; otherwise, they must petition and respond through their curators.
Source: Code of Canon Law (Vatican.va)