Does a bench warrant expire after 18 years for armed robbery?
No. Once issued a warrant does not expire. Likewise, the statute of limitations for a person to be tried for a crime does not apply when a valid warrant is in affect.
They don't actually "expire." After a number of years (usually ten) they fall out of the computer system. When that happens, the judge can either renew the warrant and keep it in the system so police can still access the information. If the judge decides to not reissue the warrant, then it is still active but police have no way of knowing about it.