I have had 4 batches of chicks and they always starting laying around 18 - 20 weeks. My first batch my husbands Grandfather would phone me everyday as soon as they were close to laying and told me to start giving them warm water a few times a day depending on how cold it was outside. I didn't want to give them laying ration too early so I listened to him and have had very good success with it. It takes devotion but works like a charm for me. My last batch started laying at 17 weeks and my Turkeys started at 26 weeks. I know lights have alot to do with it and because I live in a very cold climate they usually already have a red heat lamp on when it gets close to their laying time.
