In a prosecution for an offence against this part, if the prosecution intends to prove that the offence was committed against a person younger than 18 years old—
(a) the allegation that the offence was committed against a person younger than 18 years old must be stated in the charge; and
(b) the prosecution must prove that the defendant intended to commit, or was reckless about committing, the offence against a person younger than 18 years old.