It depends what you mean by full-grown. Physically, dogs usually attain their adult size by 9 months to a year. Mentally, some dogs have an extended adolescent phase (as long as 2-3 years) - terriers, labs and some hounds and will show puppy-like behaviors for a long period of time. I know with our two (bull/lab mix) we saw puppylike behavior well into the second year of life. Then, all of sudden, poof, they calmed down and acted like adults!