This varies from dog to dog. I raise and prepare dogs for Law Enforcement. I have waited until two years of age to relinquish dog to LE school, I have had dogs ready at 18 months, and right now I have a dog that is ready at one year of age. There is no demarcation point for going from puppy to adult, and you certainly can't use human criteria,( as so many do) to make this judgement.
I would safely say that up to a year of age a dog is still a puppy.
I had one WGWL male we actually thought was just not very bright until he hit somewhere between two and three. He turned into the mature man of the house, super dependable wonderful dog. It was like flipping a switch.
Bloodlines have a lot to do with it. I think the ASLs probably mature earlier than European lines. I had a goofball WGSL male, he was forever doing dog jokes, he was a full five years old before he was a mature adult. He did some nice bite work, though.
Some breeders have bred for oversize and I suspect they're putting the faster maturing lines together. Too much growth too soon wreaks havoc on the joints.
I'm a little surprised that a K9 was mature enough to handle the work at a year. Would love to know the breeding.
A dog is not really a puppy after a year, although many owners would like to think their dog is still a pup. Maybe, not fully mature, but not a puppy either.
I've selected and trained many dogs at 10 months to a year old, mostly Malinois and X's. But, I have had GSD's ready to go at 12 months and make it through training and are very sound Patrol dogs. When testing a young dog I look for slightly different things, I look for confidence over civil aggression. The testing is similar, but I absolutely take age into account. I want a 18 -24 month old dog to try and bite me for real, with serious aggression when back tied and I have no equipment. A 10 month old that stands his ground and is very confident is a good response for me.
Boomer was doing a full IPO 1 routine with a serious edge and outing by 9 months. He would bring it seriously and confidently at 12 months, he is not your average GSD though.
While I agree maturity can't be forced (I am of the opinion that there are 3 components making up maturity: mental=knowledge, emotional =ability to understand and contain or appropriate correct response, and physical =ability to handle and or execute tactical moves as trained) BUT the glue that holds these three components together and forms a functional and safe dog/horse/human is maturity... And that cannot be made to happen on Amy time schedule..
My girl in most ways was 'mature' very young, although as she hit 2.5yrs I saw greater discernment with her protective nature... While I knows can have the mental and emotional maturity and fortitude for LE, SAR, etc, I do wonder about the physical in the growth aspect... For SAR trailing, my girl pulls so blinkin hard and fast that I (am in greater shape then ever before) ran with her for the first 6mos so as not to put too much strain through her back and shoulders.. I want her sound her whole life if able...
My boy (a gsd/malamute) is just shy of 3 (March) and is only now showing the emotional and mental maturity a good SAR dog needs, but he physically was more than capable of miles upon miles of trails.. Lots to consider