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No, it's for all votes. So unless you want some of your voting power going to waste, wait until comments/posts are 15 minutes old.

The voting window only applies to 25% curation rewards. If it is not functioning correctly than there is another bug in the code.

Look Under Curation Updates here for the information:
https://steemit.com/steem/@steemitblog/steem-velocity-hardfork-hardfork-20

Yes, that's correct. You still waste a lot of voting power if you vote on posts/comments during the first few minutes after it was published.

When the original post came out I thought it was all votes before 15 minutes. But many said it was just for the self vote.
Here is @themarkymarks take on it. here

I believe they changed it to prevent people from using large upvotes to get the curation for themselves. It didn't matter whether they were self-upvoting or using bots to do it. The author was able to manipulate it so that they got the curation. Now it's burned. It seems it would make more sense to just get rid of this time limit. A vote should just be a the same no matter when it's cast. But then I also don't know why they get burned when voting on a post more than 7 days old.