There are so many variables. I.E. what breeds (some lay early, some lay late), and then there's the factor that chickens are individuals, even within a breed. I didn't expect one of my Light Brahmas (a late bloomer breed) to be my first layer. She laid her first egg on August 31; Buffy (the last of ten to lay) laid her first egg about 6 weeks later.
No guarantees but at this time of year if you add light to go to 12 or so hours total light a day I'll bet all that are old enough will kick in after 2 weeks. The 4 monthers will be longer. I like them to wait till after 20 weeks.
