What is the punishment for one convicted of statutory rape, and what percentage of cases actually get far enough to deal out this sentence?

There are different degree's of statutory rape that carry different punishments based on the age difference between the parties involved among other things. Below is a list of the classes and punishments.

The second half of your question would take a lot of time to research. In general, Missouri does prosecute this crime. It would be difficult to know how vigerously a specific DA's office investigates charges, but if a complaint is filed, the DA generally will investigate. For more information on the number of cases in Missouri, Go to

http://blogs.kansascity.com/crime_scene/sex_offenses/index.html

PUNISHMENT

Unless specifically stated in the statute, the following are the terms of imprisonment applied to Missouri Sex Offenses in Section 566 of the penal code:
(1) For a class A felony, a term of years not less than ten years and not to exceed thirty years, or life imprisonment;
(2) For a class B felony, a term of years not less than five years and not to exceed fifteen years;
(3) For a class C felony, a term of years not to exceed seven years;
(4) For a class D felony, a term of years not to exceed five years;
(5) For a class A misdemeanor, a term not to exceed one year;
(6) For a class B misdemeanor, a term not to exceed six months;
(7) For a class C misdemeanor, a term not to exceed fifteen days.

Section 566.032 Statutory rape, first degree
1. A person commits the crime of statutory rape in the first degree if he has sexual intercourse with another person who is less than fourteen years old.
2. Statutory rape in the first degree is a felony for which the authorized term of imprisonment is life imprisonment or a term of years not less than five years, unless in the course thereof the actor inflicts serious physical injury on any person, displays a deadly weapon or dangerous instrument in a threatening manner, subjects the victim to sexual intercourse or deviate sexual intercourse with more than one person, or the victim is less than twelve years of age in which case the authorized term of imprisonment is life imprisonment or a term of years not less than ten years.

Comment: Neither force nor lack of consent are required elements of this crime. Sexual intercourse with any minor under fourteen years old is by definition statutory rape.

Section 566.034 Statutory rape, second degree
1. A person commits the crime of statutory rape in the second degree if being twenty-one years of age or older, he has sexual intercourse with another person who is less than seventeen years of age.
2. Statutory rape in the second degree is a class C felony.

Comment: If the offender is over 21 years old and has sexual intercourse with another person who is less than seventeen, that person commits statutory rape. If the offender has not reached 21, the victim must be at least 14 years old to avoid prosecution under this law.