Puppyhood & Kittenhood
There is no binary figure or calculation that can tell you for certain the age at which your puppy or kitten is considered to be an adult dog or cat, or when they will themselves feel like an adult dog or cat, or be viewed as such by other dogs and cats. The development from puppy and/or kitten to adult is not necessarily linear, and it will pass through a range of stages that each individual dog and cat will reach at different points of their lives.
The following information is meant as a general guideline, and has been researched from other sources.
During the first twelve weeks of your puppy’s life, they are growing and developing at the fastest rate, and physical and mental development slows down after this stage. Formative points of the young puppy’s life include gaining their eyesight and hearing, beginning to eat solid foods, moving away from their beds to go to the toilet, and beginning to pick up the very basic rules of etiquette when it comes to their interactions with their dam, littermates and people.
Kittenhood lasts from birth to one year. The all-important first six weeks in a cat's life will do much in determining his personality and character for the rest of his life. Healthwise, this period is also extremely important to the developing kitten, as very young kittens are susceptible to a number of serious threats, such as fleas and URIs (Upper Respiratory Infections), which combined with other problems can lead to Fading Kitten Syndrom, a serious and often fatal condition. Kittens will probably never grow again at the remarkable rate they accomplish during this period, and seeing the changes in their development from week to week is an incredible experience.
Kittens start developing their social skills from around seven to twelve weeks, by observing their mother, by play with other kittens and cats, or in a one-cat family, by playing and interacting with their humans. This period of time will be immensely enjoyable, both for you and for your kitten, as he practices running, jumping, stalking and pouncing. Kittens at this age love to play games of "hide and seek" with their humans, and open paper bags (handles removed) or cardboard boxes make great accessories for interactive play. Your kitten will continue to grow rapidly during this period, and his motor skills will continue to develop as he practices chasing and catching "prey." He will also start adopting "adult" sleeping habits instead of just flopping wherever he happens to be.
From twelve weeks onwards, your puppy continues to grow physically, but the biggest leap made during this time is the process of socialisation, learning about other dogs and people, and finding out about the wider world. At this age, your puppy should be allowed to socialise with both other puppies and adult dogs as much as possible. Adult dogs, even those that are strangers to your puppy, will be much more forgiving of the young puppy, and give them much more leeway and room for error than they would do for another fully grown dog.
Somewhere around four months, your kitten may start losing his baby teeth, as the adult teeth develop. His gums may be painful, and this would be an excellent time to start a program of dental care, by gently massaging his gums with gauze. Plastic drinking straws are also a proven aid to teething and make for great interactive play with your kitten. Kittens will start establishing their place in the "social ranking order" of your house during this time.
It is not unusual to see a kitten "challenge" the alpha cat, which usually will earn the hapless youngster a cuff on the ears. Other cats, depending on their own social position and personalities, may defer to the kitten. Your kitten is still growing during this time, and it is not unusual to see a previously plump fluffball of a kitten suddenly grow long and lanky - then taller - and finally flesh out again. Kittens should continue to eat kitten food during this phase of growth - they need the additional nutrients for strong bones, healthy teeth, and supple muscles.
Adult dogs that play and interact with your puppy will begin to teach them valuable life lessons about appropriate play and behaviour with other dogs, who is the boss dog, when to stop playing, and how to read the warning signs and indications that other dogs give out. Play with other puppies and young dogs of a similar age is also an important bonding opportunity for your young dog, and contributes to their social development.
Your kitten is starting to show the physical and social traits of a fully grown cat anywhere from six to twelve month. No wonder - by the age of 12 months, he will have attained the physical growth of a 15-year-old human teenager, and he will undoubtedly start showing some of the same personality attributes of that age.
Don't allow yourself to brood over hurt feelings if your kitten doesn't seem as responsive to you. Like a human teenager, he is testing the waters of adulthood to see what it feels like. He is also playing a "dominance" game with you, just as he might with another cat or kitten. Be patient with him and give him all the affection and love he will take, but do it on his terms. I guarantee that he will come around when he is ready to stop playing "big guy."
Your feline youngster will continue to grow and develop for another year, and some breeds (Maine Coons are a notable one) are not fully developed for four years.
Just as with teenagers and adolescent humans, reaching sexual maturity is a stage rather than a transition, and one that takes place over a period of time. It would be wrong to assume that once your puppy or kitten reaches sexual maturity and is capable of physically reproducing that they are now adults, as they still have a lot of learning to go!
Along with the development of the sex organs, your kitten and puppy’s temperament and personality will also be growing, aided by the oestrogen, testosterone or other hormones that are circulating throughout their bodies. This adds an additional angle and stage to the process of reaching maturity, and generally begins at around six months of age.
Just because your puppy or kitten appears to have finished growing or is almost full size, do not assume that this means that they are an adult! The growth to adulthood is not just physical, but also sexual and emotional, and while reaching full size means that your puppy is physically fully grown, they may still have some way left to go!
Different breeds and types of dogs grow physically at different rates, too; small dogs such as the Chihuahua may well be fully grown by around six months of age, while for some giant breeds, they may continue to grow and develop until they are eighteen months old.
As puppies go through the final process of the transition to adulthood, they will begin to find that other dogs begin to view them as adults, not puppies, and will begin to treat them as such, giving them less leeway in games and being less tolerant of bad manners. This is one of the final stages of puppy/adolescent development, and one that again provides a steep learning curve for your young dog.
Once your dog is sexually mature (and potentially has been spayed or neutered), physically fully grown, and is emotionally mature and responsive to training and understands that other dogs view them as an adult, they are generally considered to be fully grown. This may happen by one year old, or may take up to five years old for some dog breeds and types!