I can't see any reason for this limit - in fact it seems likely to have a negative impact on the quality of information on this site. Why would you edit a comment unless to correct a mistake?
No doubt there is some reason for it, but 5 minutes still seems like too short an interval.
More than once I've spotted an error or typo in a comment I made and wanted to correct it but could not because of this 5 minute rule. leaving another comment to point out the error seems like a rather inelegant solution.