FACT: Various federal crimes have different statutes of limitations (SOL). Charges are not filed on crimes clearly barred by SOLs because SOLs are an affirmative defense. (However note it can be waived if not raised); Also note There is no statute of limitations for federal crimes punishable by death, nor for certain federal crimes of terrorism, nor, for certain federal sex offenses.

The phrase “statute of limitations” refers to the time period within which formal criminal charges
must be brought after a crime has been committed. “The purpose of a statute of limitations is to
limit exposure to criminal prosecution to a certain fixed period of time following the occurrence
of those acts the legislature has decided to punish by criminal sanctions. Such a limitation is
designed to protect individuals from having to defend themselves against charges when the basic
facts may have become obscured by the passage of time and to minimize the danger of official
punishment because of acts in the far-distant past. Such a time limit may also have the salutary
effect of encouraging law enforcement officials promptly to investigate suspected criminal
activity.”Therefore, in most instances, prosecutions are barred if the defendant points out that
there was no indictment or other formal charge within the time period dictated by the statute of
limitations. Statutes of limitation are creatures of statute. The common law recognized no period of
limitation.


quoted from  Charles Doyle, Statutes of Limitation in Federal Criminal Cases: An Overview

(October 1, 2012)