There is a statute of limitations. Generally for felonies, it is three years. However, there are exceptions.
(1) Any offense a material element of which is either fraud or a breach of fiduciary obligation within one year after discovery of the offense by an aggrieved party or by a person who has a legal duty to represent an aggrieved party and who is himself or herself not a party to the offense, but in no case shall this provision extend the period of limitation by more than three years. As used in this subdivision, the term "person who has a legal duty to represent an aggrieved party" shall mean the attorney general or the prosecuting or circuit attorney having jurisdiction pursuant to section 407.553, for purposes of offenses committed pursuant to sections 407.511 to 407.556; and
(2) Any offense based upon misconduct in office by a public officer or employee at any time when the defendant is in public office or employment or within two years thereafter, but in no case shall this provision extend the period of limitation by more than three years; and
(3) Any offense based upon an intentional and willful fraudulent claim of child support arrearage to a public servant in the performance of his or her duties within one year after discovery of the offense, but in no case shall this provision extend the period of limitation by more than three years.
4. An offense is committed either when every element occurs, or, if a legislative purpose to prohibit a continuing course of conduct plainly appears, at the time when the course of conduct or the defendant's complicity therein is terminated. Time starts to run on the day after the offense is committed.
6. The period of limitation does not run:
(1) During any time when the accused is absent from the state, but in no case shall this provision extend the period of limitation otherwise applicable by more than three years; or
(2) During any time when the accused is concealing himself from justice either within or without this state; or
(3) During any time when a prosecution against the accused for the offense is pending in this state; or
(4) During any time when the accused is found to lack mental fitness to proceed pursuant to section 552.020.
Talk to an attorney about the facts of your case.
Never describe your facts in an online forum. My statements are my opinion solely based on the information provided, and that opinion can be wrong if your facts are different than what I believed them to be. If you have any further questions, you can contact me at 636-532-1400 or through my website http://mcmichael-logan.com
Mr. Caldwell's answer would apply to most crimes, but unfortunately for the person you're asking about, there is a special statute created just for this type of situation:
"556.037. Notwithstanding the provisions of section 556.036, prosecutions for unlawful sexual offenses involving a person eighteen years of age or under must be commenced within thirty years after the victim reaches the age of eighteen unless the prosecutions are for forcible rape, attempted forcible rape, forcible sodomy, kidnapping, or attempted forcible sodomy in which case such prosecutions may be commenced at any time. "
So basically the 21 year old you mentioned is in jeopardy of being charged with statutory rape in the 2nd degree (a class C felony) until the 16 years old's 48th birthday, assuming the encounter was otherwise consensual, or at least non-forcible.
Unless and until charges are brought, the best advice would be not to talk about it to avoid drawing the attention of law enforcement or the local prosecutor. If you start to suspect that you're about to be picked up for it, that's when you'll want to probably hire a lawyer. My hope for you or whomever you're referring to is that if they haven't picked it up within 5 years and if you stay out of trouble generally speaking, the authorities might overlook it or forget about it. It wouldn't surprise me if this never gets charged, especially since the difference between this being a class C felony and completely legal was somewhere between a second and a year (when either she would have turned 17 or he would have still been 20).
Personally, if I were the prosecutor I wouldn't be that interested in prosecuting this case unless the family of 16 year old raised a major stink about it. To the extent you're not on the authorities' radar about this, the best advice I can give is to keep it that way.
I realize that I've been alternating between referring to "you" and "the 22 year old" above. I don't know whether you're asking for yourself or a friend (and it's a good idea to keep it that way). I hope I didn't confuse you with my inconsistency.
This answer is provided for educational purposes only and is not intended as the practice of law in any jurisdiction in which I am not licensed. The answer does not constitute legal advice nor does it create an attorney-client relationship, nor should such a relationship be presumed until a written agreement has been signed to that effect by both parties. The answer is based only on the information provided, and may be inaccurate in the context of additional facts that have not been provided. The questioner should be aware that I am only licensed to practice law in the State of Missouri. Accordingly, before taking any action or refraining from taking any action, the questioner and any other interested party should consult with an attorney licensed to practice in his or her jurisdiction. This information is not intended as an advertisement. However, it should always be noted that the choice of a lawyer is an important decision and should not be based solely on advertisements.