Captioning old photos?

How far can you go back into photo’s to add a caption

You can add a caption to any photo, but if you’re interested in votes, you will probably want to add captions to today’s photos, as these receive the most attention 

Thanks Chris very helpful

Is there a case for preventing new captions on a photo 3 days after the photo has appeared? Even if someone posts an amazing caption it won’t get any votes because the consensus view is that the competition is over.

Is there a case for preventing new captions on a photo 3 days after the photo has appeared? 
That would be part of the new competition format I’m pondering: https://www.caption.me/forum/discussion/2118/sealed-bid-caption-submission#latest