The juvenile court applied the wrong standard when it denied the minors’ requests for a determination of incompetency because there was no evidence of mental disorder or developmental disability. The two minors were ages 11 and 12 when they were charged in juvenile court. In this writ proceeding, both contended that the juvenile court erred in denying their requests for a determination of incompetency, because the court’s ruling was based on the ground that neither minor had a mental disorder or developmental disability. They argued that under Rule 1498(d), a minor may be found incompetent on the basis of developmental immaturity alone. The appellate court agreed with the minors and issued writs of mandate directing the juvenile court to vacate its previous rulings and reconsider the minors’ claims. Rule 1498(d) does not require that a minor have a mental disorder or developmental disability before the court may hold a hearing to determine whether the minor is incompetent to stand trial. The test is whether the child is capable of understanding the proceedings and cooperating with his attorney. An adult’s incompetence to stand trial must arise from a disorder or disability which limits his ability to understand and assist in the proceedings. The same may not be said of a young child whose developmental immaturity may result in trial incompetence despite the absence of an underlying mental disorder or developmental abnormality.