The earliest I have had a chicken lay was my australorp who was 4.5 months and the latest was my delaware and barred rock who were over 8 months. It just depends on the bird. If you are ever curious when your bird is getting close to laying @pitbullmomma, check out my article.
https://www.backyardchickens.com/articles/chickens-getting-ready-to-lay.72752/
https://www.backyardchickens.com/articles/chickens-getting-ready-to-lay.72752/