A crime that was committed more than a specific quantified number of years ago may be subject to a statute of limitation which is basically a clock making sure prosecutions don’t move forward in an attempt to prosecute a crime based on physical evidence or eyewitness testimony that may have deteriorated in its reliability over the passage of time.
After the time period of the particular statute has run out, the accused, for all intents and purposes cannot be prosecuted for the alleged crime.
Certain crimes do not have a statute of limitation. A criminal homicide, for example, has none. Some states vary in which crimes are covered by this statute such as various violent crimes, sex offenses involving minors, kidnapping, arson, forgery and other offenses.