First bitch (ASL) did hers at 1 year, easy pass. Second bitch (ASL) did hers at 5.5 months, easy pass. Dog (ASL) did his at 8 months, almost failed on the stay. Both bitches could have been tested earlier, as could the male if I'd put in the work.
My latest bitch came home at 5.5 mo, I am scheduled to have her tested when she is 6.5 mo (Czech/ASL).
CGC is VERY easy to pass with basic training. The 'trouble' spots for mine in training were stays and not freaking out when I went out of sight. The former is easy to work on. Train daily (short sessions, a few minutes) and reinforce often. Incorporate training into your games. The latter improves as you extend the time away for your pup in another room, in his crate, etc. As long as your dog is stable and you have a decent evaluator there's no reason a puppy cannot easily pass with simple daily training.
Individual lines moreso than types mature at very different rates. My females don't even start to bulk out until after they turn two, and my male achieved full size at under a year. I like mine to be slow-maturing.
I have a crazy SA labradoodle that I could probably pass CGC with, but he's not a CGC so I don't feel comfortable getting him the certificate. He wouldn't deserve it. :P