Duke was a few months shy of 3yrs old. I got him at 2yrs old, and they were holding it at our training group.... so we joined in. He's a german show line, neutered male.
Storm could pass right now (if she behaves!), but we're focusing on getting her BH currently. She's 1.5yrs old, west german working line, intact female.
I do believe some lines are slower to mature. I see all the time at training and with my own dogs. I've seen certain litters just start to mature at 3, and others are way advanced in maturity. Storm is fairly mature for her age, so was Duke. When we got him at 2, his maturity was more of a 4-5 year old.... not a dog just turning 2. Zira on the other hand is still all over the place and she's almost 3. Body and mind... I believe they all mature at a different pace, but certain lines also tend to sway that a bit.
I never officially did it with other breeds, but I did train my goldens back in the day, and I'd say knowing what they do during this tests... my goldens would have been fine at about 1.5yrs because they were a little hyper and the more sporty type. I've seen a lot of the calmer breeds pass at 8months and below. We did a mock one a few months ago for everyone at training, and we had a 6 month old join for the experience.... they would have passed right then and there if it were the real thing. Depends on the dog.... it's all really basic though.
The CGC was fun, I didn't sweat it.... we just had a good time. If anything, doing a "mock" CGC never hurts for any questionable or possible problematic areas.... it also helps with any handler nerves or concerns.