Well, you can enter your dog in an AKC Herding Instinct Test at 6 months of age. I'm not sure what is considered an optimum age to try. My girls were both 2 years old when they took the test. One passed both tests and got her Herding Instinct Certificate, the other just passed one test (chased and bit a sheep on the second test).
~ Diane ~
SCARLET ..... Lauremi's Almost Wasn't
CARLY ......... Ch. Lauremi's No Reservations (AKC GCh pointed, HIC)
RUSSELL ..... Lauremi's Paix and absent friends... SAGE ~ Lauremi's Whim Z v Jakmar ~ AKC major ptd, HIC ~ 2010-2015
I did it at around a year...bunnies are too much prey and not enough herding. I did it around a year so that my dog had enough obedience on him that if he decided to do something bad I could call him off.
Don't equate how your dog does with bunnies to how he will do with sheep. Sheep are bigger and in general more fun to chase than to eat. I think you can go ahead and do it now if you want...I never went on to train in herding (too far away) but I'm not sure how long it takes for your dog to advance from sheep to something smaller. I'm not sure if bunny herding is even possible. I know ducks and geese are sometimes used but if you intend for your GSD to herd bunnies it might not ever happen.
Stosh started herding training at 10 mos, but you can take the AKC test at 6 mos. It consists of the dog on a long lead in a pen with the sheep and you and the trainer. The dog has to show interest in the sheep and basically move them around the pen. The dog shouldn't be afraid of them or try to attack. Many dogs will bark or lunge or even nip. The trainer will be able to tell if it's interest or aggression. You can go to AKC.org and search for upcoming events in your state