Welcome to BYC. What breed are your chickens? Different breeds of chickens can be different sizes, and will lay at different ages. Most chickens don't start laying until after 20 weeks, except the high production birds who can start near 16 weeks. Their combs may start reddening-up a few weeks before starting to lay. When chickens come into lay this time of the year when the daylight hours are short, they may not start laying until the days get longer. It is not good to treat with antibiotics unless they have an infection, and you know what you are treating. Chickens who have coccidosis as chicks can have intestinal damage that can stuck their growth and decrease egg production.