I'm at 495 and can't downvote.

Quick, 5 people upvote me and I'll tell you if it kicks in.

OK, at exactly 501 you get a little down arrow (doesn't appear at 500).

dang already confirmed it below, but at least I got to feel like a camgirl for 5 minutes.

Thanks for playing!

Would you like to know whether you lose your downvoting ability if your karma drops below 500 again? Just ask, and I'm sure the HN crowd is willing to help you find out :-)

Also, would that freeze the downvotes you gave earlier, or would you still be able to undo them?

We can all dream

I got my downvote ability at around 550 or so...

For me it was 500.

I'm not sure the number is constant. I think it scales with either the age or the prominence of the comment. I've seen threads where I was unable to downvote top tier comments, even though I could downvote their children.

It's constant at 501.

I did see interesting behavior when I crossed the threshold: I couldn't downvote comments that existed before I reached that karma level (or at least the UI didn't render them with a down vote button).

Downvoting wouldn't be affected by that per se, but downvote arrows last for a much briefer time window than upvote arrows do.

I think after 8 hours you can't downvote a comment anymore.

Are you sure you do not suffer from a mitm?

I have 117 and can downvote.

Are you certain? Maybe you are thinking of the 'collapse' button, which is represented by a minus sign?

Yep, although it is not near the upvote button and sits among such links as parent and flag.

500 AFAIK

400 or 500 points, I forgot which. Maybe someone with something between those two can tell?

Don't know for sure but empirically it's more than 450.