A lot of the questions I get from puppy owners come down to expectations that don’t match the dog’s age. A 4-month-old Shih Tzu that still has accidents. A 9-month-old Labrador that suddenly ignores its name. A German Shepherd that is full size at 15 months but still acts like a puppy. In most of these cases nothing is wrong with the dog. It is simply in a stage that comes with that behavior.

Knowing which phase a dog is in makes it easier to set the right goals and the right amount of exercise, and it takes away a lot of frustration with a dog that is doing what dogs that age do. Below is the timeline, what happens in each phase, and how size shifts the timing.

The Development Timeline at a Glance

The ages below are averages. Individual dogs vary, and mixed breeds like most Aspins can land a bit earlier or later.

• Neonatal (birth to about 2 weeks): The puppy depends completely on its mother. Eating, sleeping and growing.
• Transitional (about 2 to 3 weeks): Eyes and ears start working. By 3 weeks they are well developed, and the puppy begins to notice the people, dogs and surroundings around it.
• Socialization window (about 3 to 14 weeks): The period when puppies learn what is safe and normal. Experiences here leave a lasting impression. A first fear period often shows up around 8 to 10 weeks. Most puppies should stay with their mother and littermates until around 8 weeks.
  • If a breeder is selling puppies younger than 8 weeks, walk away. Either it is not a legitimate breeder, or the welfare of the puppies is not their priority, or both.
• Juvenile (about 3 to 6 months): More confidence, more chewing, testing limits, and usually sleeping through the night.
• Adolescence (about 6 months to 1.5–2 years, depending on size): Puberty starts, usually between 6 and 9 months in males and between 6 and 16 months in females. Around 8 months many dogs go through a phase where they listen less to their owner. A second fear period can appear somewhere between 6 and 14 months, usually lasting one to three weeks.
• Social maturity (about 12 to 36 months): Behavior settles and the adult personality shows.
• Adulthood: Physically, small dogs are done growing well before their first birthday and giant dogs closer to two. Behaviorally, most dogs count as mature adults from about 2 years.

Puppyhood: Birth to About 6 Months

Body and brain. This is the fastest growth of a dog’s life. The brain develops quickly too, through at least 20 weeks, and what the puppy experiences now shapes how it reacts to the world later.

Typical behavior. Curious, mouthy, short attention span, lots of sleep. Between 3 and 6 months the puppy gets bolder and starts checking what it can get away with. Chewing goes up.

What to focus on. Socialization comes first. This is when the puppy learns what is normal and safe, so calm, positive meetings with people, friendly vaccinated dogs, sounds, surfaces, car rides and being handled matter more now than later. Always go at the puppy’s pace. Next to that, the basics: a potty routine, its name, sit, coming when called, walking on a leash and settling on its dog bed. Short sessions, many times a day. During the 8 to 10 week fear period, don’t force anything scary.

Adolescence: About 6 Months to 1.5–2 Years

Body and brain. Puberty and hormones. Small dogs finish growing early in this phase, while large and giant dogs keep growing through all of it. The brain is still maturing: impulse control, flexible learning and attention to people all improve a lot between puppyhood and about 21 months. Behavioral development continues until roughly 2 years.

Typical behavior. This is the teenage phase, and it is real. In a study of young guide dog candidates, dogs at 8 months were more likely to ignore “sit” from their own carer than they had been at 5 months, while they actually responded better to a stranger. The dip passed, and it was stronger in dogs that had shown signs of a less secure bond with their carer. Expect more independence, more interest in smells and other dogs, sometimes a new fear out of nowhere. Big dogs often don’t know their own size yet and bump into people and furniture.

What to focus on. Don’t take it personally, and don’t stop training. Stay consistent, keep rewarding the behavior you want, and make exercises easier for a while if needed. Harsh handling works against you here, because the dip is bigger when the bond is less secure. Keep socializing too: calm, positive experiences through adolescence build confidence for adulthood. Use the leash wherever a recall can fail. For exercise, think sniffing walks, training games and free play rather than distance, especially for large and giant breeds.

Adulthood: From About 1 to 2 Years Onward

Body and brain. The bones have finished growing and the dog has reached its adult height, although it can keep filling out with muscle for a while. Social maturity lands somewhere between 12 and 36 months, and dogs between 2 and about 6 years are considered mature adults. Senior years usually start around 7.

Typical behavior. More settled. Excitability is highest in puppyhood and goes down steadily with age, while attentiveness to people peaks in early adulthood.

What to focus on. Once the bones have finished growing, running and dog sports can be built up gradually. Walks can get longer and faster. Keep training: adult dogs learn just fine, and a calm, confident, polite dog is the result of ongoing practice.

How Breed Size Changes the Timeline

Small dogs finish growing at roughly half the age of big dogs. Bigger bones simply need more time.

Size (adult weight)	Examples	Full adult size	Exercise while growing
Toy / small (about 2–11 kg)	Chihuahua, Pomeranian, Shih Tzu	About 6–12 months	Bones finish growing early, around 6–9 months in toy dogs. Self-paced play until then.
Medium (about 11–27 kg)	Beagle, most Aspins, Shar Pei, Siberian Husky	About 12–15 months	Self-paced play while growing. Build up running gradually once growth is done.
Large (about 27–45 kg)	Labrador, Golden Retriever, German Shepherd, Belgian Malinois	About 15–18 months	Bones can keep growing until 16–18 months. No jogging, road work or repeated jumping before then.
Giant (45 kg and up)	Great Dane	About 18–24 months, some later	Growing for up to 2 years. Same rules as large breeds, for longer, before any running or jumping sports.

Growing Joints and Exercise

Young dogs grow from soft areas near the ends of the long bones. When growth is finished, those ends harden and the bones stop getting longer. Until then, long, repetitive running on hard surfaces like concrete can damage growing joints, and this matters most in large and giant breeds. Self-paced play with lots of turning and stopping is the better choice, ideally on grass or sand, and so is play with well-mannered dogs. A puppy that runs and stops on its own terms is fine. A puppy jogging next to a bike is not.

Two Walks a Day, Adjusted to Age and Size

Two walks a day is the routine for every dog, whatever the size or age. What changes is the length and the pace. For puppies and adolescents, these are sniffing walks on a leash: an easy pace, time to explore, calm meetings with the world and practice walking nicely. Large and giant breeds get the same two walks while growing, just not long distances or fast pace until the bones finish growing. Adult dogs can handle longer walks with more pace.

Mental Maturity Lags Behind Size

This works both ways. A 9-month-old Pomeranian may be fully grown, but it is still going through puberty and teenage behavior. A 14-month-old Labrador or German Shepherd looks like an adult dog, yet behaviorally it is still a teenager in a big body. A 1-year-old Great Dane is a giant puppy. Judge a dog by its age and stage, not by its size.

Where to Go From Here

Most “problems” in the first two years are normal stages, and every stage asks for something different from the owner. For a step-by-step version, follow the Puppyhood week-by-week journey. It goes from the newborn weeks through the teenage stretch and into adulthood, with what to do at each point.

Sources

• VCA Animal Hospitals, New Puppy Guide: stages of puppyhood
• VCA Animal Hospitals, Puppy Behavior and Training: Socialization and Fear Prevention
• Purdue University Center for Canine Welfare, Puppy Developmental Stages
• AVMA, Literature review: Socialization of puppies and kittens
• AKC, When Do Dogs Stop Growing?
• AKC, How to Care for Your Large-Breed Dog
• PetMD, When Do Dogs Stop Growing?
• Veterinary Partner (VIN), Nutrition and Exercise for Growing Puppies
• Asher L, England GCW, Sommerville R, Harvey ND (2020). Teenage dogs? Evidence for adolescent-phase conflict behaviour and an association between attachment to humans and pubertal timing in the domestic dog. Biology Letters 16: 20200097.
• Harvey ND (2021). How Old Is My Dog? Identification of Rational Age Groupings in Pet Dogs Based Upon Normative Age-Linked Processes. Frontiers in Veterinary Science 8: 643085.
• Bray EE et al. (2021). Dog cognitive development: a longitudinal study across the first two years of life. Animal Cognition.