My wife used to work with a woman who used to breed Labradors for the blind as seeing-eye dogs. She said that they would start weeding through them at 6 weeks and up. There are certain benchmarks along the way that the dogs have to meet, otherwise they are going to wash out anyway, and it isn't worth the investment of time to try to train them beyond that. But, they can still make fantastic pets! Interestingly, it seems that the chocolate labs wash out at a drastically higher rate than the blacks or yellows... Something about their genes makes them a bit more high-strung, apparently. They all still get a shot, but the chocs just don't seem to be cut out for that style of life. Great pets though!