The Breed Standard
Labrador retrievers reach physical maturity between 11 and 15 months of age. Most dogs gain weight fairly steadily throughout the earlier part of their growth phase, until they reach about 65 percent of their adult weight. The American Kennel Club's breed standard describes the "ideal" adult male lab as weighing between 65 and 80 lbs.; adult females should weigh 55 to 70 lbs. Most lab puppies will weigh about 50 lbs. at 7 months of age.
Every Dog is Different
Not all labs--in fact, relatively few labs--are bred to compete in dog shows. Many adult labs are much larger than the breed standard, and some are smaller. And individual dogs mature at different rates--some labs will reach full size at 10 months of age, others won't be fully grown until they are 15 months old or older.
Weight is largely a question of genetics. The best estimators of a puppy's weight at a particular age are the weights of its parents and litter mates at the same age.