Mo. Rev. Stat. § 566.020: Mistake as to age — consent not a defense, when.

1. Whenever in this chapter the criminality of conduct depends upon a child being less than fourteen years of age, it is no defense that the defendant believed the child to be older.

2. Whenever in this chapter the criminality of conduct depends upon a child being less than seventeen years of age, it is an affirmative defense that the defendant reasonably believed that the child was seventeen years of age or older.

3. Consent is not a defense to any offense under this chapter if the alleged victim is less than fourteen years of age.