Either they tweak the threshold over time, or there are other factors than karma. I'm sure I gained a downvote button well before 1000. I don't think it's particularly useful -- I only use it when I think a comment isn't worth reading at all. I usually upvote people when I think they're wrong, so we can have an interesting discussion.

Your user page shows you how much karma you need to hit the next level. In your case, you need 172 to downvote comments. https://news.ycombinator.com/user?id=exikyut

It's 500 last time I heard, but you don't get a downvote button on every comment.

And there is no downvote button on posts (or if it is, I haven't reached that threshold yet.)

I think I got it around 500~. There could be a manual component to it?