It's a pretty tough thing to do - sexing - at any age, but if you know what to look for, you can do it as eary as they are born. I guess a lot when they are a couple days old, but usually by 3 weeks, I can tell their sexes pretty well. I've only had a few gender-benders.
Just like palpating, the more experience you have, the better you are at it.
Also, only female rabbits have teats, so it's possible to attempt sexing this way...ahah, but I'm not sure how old the babies would have to be before you can feel their teats - never tried it before.
Emily