I've been lurking for a lot longer than my account has been active (probably a good 5 years now); have never seen what the criteria are for being able to downvote. Could you please enlighten me?
It 'used to be' 500 karma. But I've heard it goes up over time. I've heard people talk about having ~2k karma without the privilege. So I don't really know to be honest. I wish somebody could answer your question more precisly.
Honestly, I'm a bit curious too, bu kinda do not want to know, as I may be then tempted to 'go for it' and obtain that ability.
It's enabled exactly when you have 500 karma. One of my colleague just got that.
~500 Karma
It has to be a lot higher than that now because the commenter with the comment above yours has ~900 karma and doesn't have downvote privilege.
I have 600 and can downvote, though. It appeared at exactly 500.