Every charge has some type of limitation.  If the charges are not brought by the time the required statute of limitation is reached, then it is subject to be dismissed upon the motion of the defendant or independent motion of the judge.  Virtually every charge has this type of limitation.  Some states have removed the statute of limitations for certain sexual offenses... usually the most horrible of sex offenses... but virutally all others have a time frame ranging from six months to fifteen years, in which a charge must be prosecuted.Keep in mind, however, that this does not mean a conviction must be obtained.  It only means that a charge must be initiated.