How Long Before a Dog Is an Adult?

Small dogs reach their adult height between 10 to 12 months of age. Small breeds are dogs up to 30 pounds when fully grown. Medium breeds reach full growth between 12 to 16 months of age. Medium breeds are dogs between 30 and 80 pounds when fully grown. Large dogs or large breeds can take as long as two years to reach their final size. Large breeds are dogs that weigh more than 80 pounds as adults.

Sexual maturity can vary among breeds. Small-breed dogs tend to mature faster than larger dogs. Most males, however, are fertile after six months of age and reach full sexual maturity at 12 to 15 months of age. Most female dogs or bitches have their first season or heat after six months of age. However, there are some dogs that may not have their first season of heat until 18 to 24 months of age.

Four to six months of age is preadolescence. Puppies become more independent and confident. Six months to 12 months old is a difficult time because your pup reaches sexual maturity, but he's still an adolescent. Like humans, he will test the waters by disregarding rules he learned earlier. During 12 months to 18 months, your pup finally reaches emotional maturity. It will be sooner with smaller breeds. Some dogs will show dominant behavior and try to be the pack leader. Consistent training is necessary.

It used to be a common denominator that a dog reached adulthood at one year of age. To emphasize this belief, dog companies still recommend you stop feeding puppy food to your dog when your pup turns 1. Of course, there is no magical number. You can tell when your pup has stopped physically growing because he doesn't grow for several months. Emotional behavior is noticeable, too. A puppy typically greets you more enthusiastically then an adult dog.