Someone mentioned selecting breeders after 2 years, That is not correct, at least for birds selected for meat duties, selecting for Body Weight Gain ay 4-6 weeks it's recommended. That is how a close line of White rocks can reach Broiler Weight gains under 6 weeks, the. Line has been closest for 50+ years
Actually, that was me regarding the two year breeder mark.
There are lots of really good reasons for holding off until two, and some of the serious breeders recommend three years. You want to make sure your birds, hens and roos, can grow to maturity without major health issues, that they are within the standard (including looks, conformation, weight, egg color, etc.) the entire time (that's where I mentioned measuring along the way), and any other qualities that you are looking for such as setters/non-setters, egg quality, personality, etc.
Dr. Temple Grandin has an excellent explanation for how breeders looking for just one or two traits can accidentally breed unwanted traits into their animals. Hip dysplasia in Labrador Retriever and German Shepherd dogs are the classic example.
If a breeder is not willing to wait to see if bad traits show up later in life, those bad traits could eventually show up earlier and earlier. If you select solely for size at 6 weeks, breed those birds at six months, repeat several generations, you could be breeding birds that are progressively smaller at maturity, and that will catch up with your flock at six weeks, eventually.
That's why "breeding" and "raising" are two different activities. Raising birds can be profitable, breeding not so much, but it is rewarding for those of us who do it. Not just the show trophies, but knowing that the entire breed of chicken is improving as a result of your efforts.