There isn't one.  Quite simply, it looks to me as if Wyoming is the only state in the country with no criminal statute of limitations at all.  Theoretically, the police could go into a nursing home and arrest someone there, for having shoplifted a piece of penny candy when they were 5 years old in 1920-something.

That doesn't change the fact that after this much time, a crime is usually much harder to prove, and the standard of proof, beyond a reasonable doubt, still applies.