There isn't a post date limitation to post you can report although there are far more recent spams to tackle that's why I'll encourage you report more recent posts as those would be more beneficial to the forum than the old posts. The goal is to end users from getting rewarded for spamming the forum and not to clean up their mess. This advice only has to do with posts considered to be spam or offtopic. Now if you see plagiarized post irrespective of the date the post was made, you should go ahead to report those and leave it in the hands of the moderators to take action.

Irrelevant mega threads turning into a spam hangout for spammers should also be reported but in this case you should not only report the spams but also report for such threads to get locked.