Neb. Rev. Stat. § 28-319.01

(1) A person commits sexual assault of a child in the first degree: (a) When he or she subjects another person under twelve years of age to sexual penetration and the actor is at least nineteen years of age or older; or (b) When he or she subjects another person who is at least twelve years of age but less than sixteen years of age to sexual penetration and the actor is twenty-five years of age or older. (2) Sexual assault of a child in the first degree is a Class IB felony with a mandatory minimum sentence of fifteen years in prison for the first offense. (3) Any person who is found guilty of sexual assault of a child in the first degree under this section and who has previously been convicted (a) under this section, (b) under section 28-319 of first degree or attempted first degree sexual assault, (c) under section 28-320.01 before July 14, 2006, of sexual assault of a child or attempted sexual assault of a child, (d) under section 28-320.01 on or after July 14, 2006, of sexual assault of a child in the second or third degree or attempted sexual assault of a child in the second or third degree, or (e) in any other state or federal court under laws with essentially the same elements as this section, section 28-319 , or section 28-320.01 as it existed before, on, or after July 14, 2006, shall be guilty of a Class IB felony with a mandatory minimum sentence of twenty-five years in prison. (4) In any prosecution under this section, the age of the actor shall be an essential element of the offense that must be proved beyond a reasonable doubt.