I can't confirm it's correct but this is from the Shouse law group in California :
In some states, mistake of age is an affirmative defense. In others, it is not.
Contrary to the law in some states, mistake of age is not an affirmative defense in California.[8] This means that the defendant does not have the burden of proving their mistake of age defense. Instead, the prosecutor has to prove, beyond a reasonable doubt, that the defendant did not reasonably and actually mistaken the alleged victim’s age.