>The vet said that it is impossible to tell at this age, it is better when they are about 12 weeks.<
12 weeks will surely be much too late and you will likely have fighting rabbits and taking a chance on unplanned pregnancies! Your vet is definitely wrong about this. Chances are, you're not pressing down on the casing to make the organ protrude enough to see it. Testicles will begin developing around 10 weeks of age... some later than others. During the fryer stages, the bucks tend to be larger than the does (the does catch up and surpass them later on).
Did you check out the web sites I listed in my previous post?
Pat Lamar
President
Professional Rabbit Meat Association
http://www.prma.org/