Unlike humans, there is no legislation that says that at insert-local-legal-age-here a dog becomes an adult. However, there is a lot of information (mostly on food) that is different depending on if your dog is a puppy or an adult.
At 11 months old, I have no idea if my dog is a puppy or not! How can I tell?
I realize that this question is similar to this one (At what age can my dog be considered old?) that was closed for being opinion based. But as I mentioned, since there are a lot of products targeted at "puppies" and at "adult dogs" I believe that there has to be a factual answer out there, at least in the eye of the food industry.