Quite consistently: no downvote button on comments more than 24 hours old; no downvote button on comments that are followups to your own comment.

Submissions don't have downvoting, but you can flag them.

The pattern I see is no downvotes to replies to YOUR comment (e.g. this one), and no downvotes once the comment is about 2 days old-ish.

There is no downvoting for articles, independent of karma, that‘s correct.

I have 7500 karma and I have downvote for comments, but not for articles. I only can't downvotes comments that are replies to my own comments(which I think is smart and is something that reddit should copy).