I definitely think outright looking for old posts to flag is usually a bad idea, but there are cases where Really Bad art may have been uploaded a long time ago, and still deserve to be flagged. The thing is, the standards for flagging should reflect what the standards for uploading were at the time the picture was uploaded. I'm not saying they should be exactly as strict or loose, but I do think that the standards of the time of the picture are more important than the present ones.
That doesn't mean all sorts of bulk flagging of old posts are inherently bad, but the standards should definitely be closer to what they were when the images were uploaded.
Super strict scrutiny should stay with recent uploads, since that's by far the most important place to put a high amount of effort, and effort isn't an infinite resource. If users put the most effort into thinking about recent uploads, like they should, then flags on old posts will be a lot more arbitrary, both in placing them and handling them.