What is the test for a defense based on mental age?

Arguments essentially identical to those raised by defendant today were presented to this court in People v. Oxnam, supra, 170 Cal. 211, 149 P. 165, and People v. Day (1926) 199 Cal. 78, 248 P. 250. In Oxnam the trial court rejected a defense based on mental deficiency; defendant then moved for a new trial, submitting an affidavit showing him to have a mental age of eight. We affirmed the trial court's order denying that motion. In People v. Day defendant also moved for a new trial based upon affidavits showing that his mental age was that of a child under 14. We responded that "Section 26 of the Penal Code, subdivision (1), is not susceptible of the constrained construction contended for by the defendant. . . . Said section clearly refers to the physical age of a Child and has no reference to the mental or moral age of an adult." (199 Cal. 78, 87, 248 P. 250, 253.)