It depends on the trainer. I did Paisley early, but I had previously worked with my trainer. I know the AHBA does official tests at 6 months (first leg of the HCT), which I did with Paisley just for fun and the experience since I already knew she had good instinct.
I have been herding with her consistently since 6 months (before that it was monthly). Now that she is about 15 months, I have noticed a big difference in her ability to listen around the stock (just noticed this yesterday). She has been getting better for a bit, but noticed a big difference when we went this week (after we took a few weeks off due to conflicts). Knock on wood that it continues...
I just had an evaluation on Lakota she's 4-1/2. That wasn't a club she was a trainer. I was a lot of fun and Lakota loves it. The following week I took one lesson with the trainer working her. I plan on doing a few more.
I did a herding instinct test on my male at 11 months and my female was 2. Both did so good, but my male completely blew the evaluator away. He was just such a natural and needed very little direction - my female thought it was fun to run into the herd and scatter them before herding them up, but she still was a great herder LOL
I definitely think it depends on the dog, there were a few corgis there that were younger than my boy that did a fantastic job
I instinct test mine at 7 weeks. My current dog was done at 16 weeks. An experienced trainer knows how to pick the right stock for a puppy [flock, no panic and honor the puppy's pressure] and not over-pressure the puppy. Find out who the trainer is and contact them regarding how they run the test and in particular how they intend to communicate with your dogs and what tools they use [long line, rake, stock stick, etc.] and how they intend to use them. I generally like to know the dog's age, degree of social and prior training. A lot of times a GSD won't work for a stranger depending on age/experience and won't accept pressure or corrections from a stranger. Here's where its important to know your dog and have a good feeling how that trainer intends to work your dog.