Labrador Retrievers generally have a higher drive to please their owners than they do prey drive. However, pups are very boisterous and can easily injure your birds. I think starting sooner is way better than later. I'd say that you could probably use some desensitization and positive reinforcement techniques to teach your pup to ignore your birds. Start off far away from the coop and with your dog on a long line. Use a toy or treats that he's absolutely nuts over. Reward and praise him when he pays no attention to the chickens. Very gradually over a period of time, and only when he is succeeding 100% of the time at that distance, move closer to the coop. You might have to switch up rewards so as to keep his attention, but always use something that he can't resist! First and foremost, though, excercise the bejesus out of your dog beforehand! A tired dog is a good dog, and so much easier to work with. Otherwise you will have tons of pent up energy that has to go somewhere, and little running critters will be target number one. Just don't forget what labs were bred to do and never leave him unsupervised around the chicken coop. With lots of consistency and positive reinforcement, you should have great success. Good luck!
ETA: if he starts barking/growling/reacting, session ends. No more treats, take him inside and away from the situation immediately. 
How close has he been allowed to get to the coop? Was he leashed? A lot of the reaction could be frustration if he has not been allowed to get up close and really check them out. Either way, desensitization is a good idea.