It probably depends on what you mean by "adult". Do you mean fully grown and feathered (about 12-14 weeks), or mature - ready to mate/lay eggs (generally 6-8 months), or mature behaviour - fully capable of taking care of goslings (1-2 years)?
My goose (not a Chinese) weighed about 12 pounds, when she started laying last year. During her first season she gradually gained weight, and now she weighs about 20 pounds. Her behaviour during that year has changed a little, too. She's a bit more aggressive towards strangers, and she rarely follows me around the yard anymore. So now I consider her an adult in all senses.
