I tried it out by promoting an older post for 21 SBD. The system seemed happy to take my money, but the post disappeared within an hour. If we are not supposed to be promoting older content, the option shouldn't be there.

But scrolling through the page, I see a few older posts by other authors. One, by @fyrstikken is 2 weeks old.

I wrote a post on this issue and promoted it for 1 SBD, it seems to be appearing just fine.

https://steemit.com/steemit/@kendewitt/can-t-promote-older-posts-with-the-new-promote-post-feature