Golden Retrievers will stop growing completely when they are 1 and a half years old, so a 10-month-old Golden Retriever puppy will still grow, but at a slower rate. Generally speaking, the bones and muscles of a Golden Retriever grow together in the first 3 months after birth, until they are 4- The body grows at 6 months of age, the height and bone mass increase at 7-10 months, and finally takes shape after 11-12 months of age. It is not until one and a half years old that the body is completely set and matures sexually.