PC 43.25(g) says that, when necessary to prove up the age element of under 18, "the court or jury may make this determination by . . . "

The obvious interpretation seems to be, "it's up to the trier of fact to decide," but the language is loose enough that I think you could make a case for the court to decide who gets to decide, in a jury case. Is there settled law on this, or am I just making a ridiculous argument?

The judge decides it in a court trial; the jury decides it in a jury trial.