I just learned what this means.
From I.D. v. Juvenile Officer, decided today by the Missouri Court of Appeals (Judge Lisa White Hardwick, joined by Judges Gary D. Witt and Thomas N. Chapman):
Doli incapax is a common law presumption that a minor between the ages of seven and fourteen lacks the capacity to commit crime.
But, the court makes clear, it doesn't prevent juvenile proceedings against the minor (and of course this isn't a constitutional mandate, so it can be changed by statute).