“Missouri, like most states, takes the difference in age between the partners into account when one or both of them are under the age of consent. In general, a person over the age of 14 can consent to sex with a person under the age of 21 without legal problems. Most crimes related to the age of consent in Missouri only come into play when someone over 21 has sex with someone under the age of 17.”
http://www.legalmatch.com/law-library/article/missouri-age-of-consent-lawyers.html
Except if one is passed out drunk then it’s illegal everywhere.