Old transcripts for Grad application
In general, a signed, sealed transcript of a completed degree program should always be valid, as there presumably there has not been any changes made to the contents of the transcript since that time.

"Interim" transcripts, of course, would no longer be valid. 

One other thing to note—many schools nowadays want the transcript submitted directly by the university; in such cases, it is unlikely that they would accept a transcript mailed by the student along with the application (if they even accept mailed applications!).