There is a stretch early in a puppy's life when its brain is unusually ready to file new things away as normal, and it does not stay open for long. Roughly between three and sixteen weeks, novelty is accepted easily; after it, the same puppy grows more cautious, and first impressions cost far more to make. Almost nothing else in raising a dog has such a short deadline.
What socialising actually means
It is not a numbers game of meeting as many dogs as possible. The aim is calm, pleasant introductions to the ordinary furniture of a dog's life — different people, floor surfaces, household noises, car trips, being handled and having its paws touched — each kept low-key enough that the puppy stays relaxed. A frightening experience registers just as firmly as a good one, so gentle and positive matters more than busy.
The vaccination puzzle
The catch is that this window closes before a puppy has finished its vaccinations, which leaves owners caught between two risks. Shutting the puppy away until it is fully covered is the more common mistake, because a dog under-socialised in these weeks can carry the anxiety for life. Carrying the puppy in public, meeting healthy vaccinated dogs on clean ground, and well-run puppy classes are the usual ways to thread it — and your vet can help you weigh the balance for your area.
After sixteen weeks
Missing the window is a setback, not a verdict. An older puppy or an adult can still learn to take the world in its stride; it simply asks for more patience, smaller steps and, for a genuinely fearful dog, the help of a good trainer or behaviourist. Earlier is easier, but later is far from hopeless.
When is the socialisation window for puppies?
Roughly three to sixteen weeks of age, when a puppy most readily accepts new things as normal. After it the same dog grows more wary, so early, gentle exposure counts for a great deal.
How do I socialise a puppy before it is fully vaccinated?
Without shutting it away — carry it in public, let it meet healthy vaccinated dogs on clean ground, and use a well-run puppy class. The behavioural cost of isolation usually outweighs the risk, but ask your vet how to balance it where you live.
Can you socialise an adult dog?
Yes, though it is slower than with a puppy. Small, positive steps help, and a fearful adult dog is worth the help of a qualified trainer or behaviourist rather than pushing it.