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I dont know the answer. I do notice that upvoting old content doesn't seem to make a difference, but I dont know the expirey date on re-steeming. For older posts people seem to include the URL in their own posts which i think is actually more valuable because they can give context to why they are re-steeming it. Kind of like the difference between that friend you have who always forwards you crap vs those who write you an email with a link and why they think its interesting.

I've noticed that after a post is a few days old to maybe a week old I no longer have the button to resteem it. I'm not sure exactly but seems after a few days old you can't resteem.

You actually brought up an interesting point, I wish they had an option to restemm with comment similar to how you can retweet with comment. Typically when we resteem something it's because we learned something or we feel there's a message worth resharing, it would be nice to be able to add our own commentary or the reason why we're resteeming it.