My leghorn laid her first egg at about 17-17.5 weeks. At about 15 maybe 14 weeks her comb started growing and got red, it happened so fast and was almost like it happened overnight. Before she started laying she would go into the coop and make alot of noise, different from her normal noises, scratch around like she was trying to make a nest, and ofcourse the squat.
In the picture, you can see how big her comb is, this picture was her at I believe 15/16 weeks, and after this picture was taken, her comb grew even more and hangs to the side like a hairdo, its absolutely adorable.
But like someone else said, they will likely start laying when you give up getting eggs.