What is the minority defense in a civil case?

lawful only on prescription, may often be practically impossible for the prosecution to prove but easy for the defendant to refute. In the absence of a legislative provision that minority is a defense, we do not believe that the relative ability of the prosecution and defense to establish the defendant's age is sufficient to justify invoking the rule of necessity and convenience to relieve the prosecution of its burden of proving the defendant's majority under [Welfare and Institutions Code] section 11502. [] The defendant may not necessarily have any substantially greater ability to establish his age than does the prosecution. A defendant's precise age is not a matter within his personal knowledge but something he must have learned either from family sources or public or church records. In this age of documented existence there is little doubt that ordinarily the prosecution may be able to secure evidence of the defendant's age. Moreover, in those rare cases where there is no evidence of the defendant's precise age except his own belief as to what it is, the defendant might be as hard pressed as the prosecution to verify it. If minority be deemed a defense, a defendant in such a case would be at the mercy of the jury's power to disbelieve his testimony even though it be the only available evidence of age. We conclude that a case for the application of the rule of necessity and convenience here has not been made out." (People v. Montalvo, at pp. 334-335.)