How would you really police this effectively though? Do you think it should be made a forum rule or just an unwritten one? Should we prohibit old posts from even being allowed to be reported in the first place by removing the report button for posts that are x amount of time old? Then what happens when there is something that should be removed like doxxing or whatnot? I agree that half of the time very old posts aren't important (especially when people are seemingly going out of their way to find minor infractions) but the only way I can think of is to make it known that it's unnecessary to report certain things, especially if they're historic cases (and for minor violations). Probably wont stop people from reporting them though. Probably bigger things to worry about as well really.
By most of the comments it is probably bad idea, with even 1% of chance to be accepted in any form - as forum rule, or just as a guideline for moderators. I was just thinking that if sometning is not reported in period of two years (and it was bad), it should stay in past - but I guess it is better to leave things as they are now.
May I suggest that someone concerned about getting caught for plagiarism committed 5 years ago should just go to their post history and remove said plagiarism. Or just say that they bought the account, that works too.
I doubt people with thousands of posts will even try to do that, it is not only time consuming, but it is hard to remove such post if you do not know that it is copy / pasted from somewhere. In that one case where user is claim he bought account with plagiarism posts, admin is step in and check his log history which proved he is not real owner of account. I doubt we will see another case like that.