Birds that have been bred for heavy egg production (like Leghorns, Red Stars, Black Stars, and other hybrid crosses) will usually start laying at about five months of age, or even 4 1/2 months. Dual-purpose breeds like Rhode Island Reds, Wyandottes, and Barred Rocks, will take a little longer on average. Some breeds, like Silkies and many bantams will take over seven months to start laying. However, it really depends on the specific bird and enviromental conditions. My Silver-Laced Wyandotte and Columbian Wyandotte started laying about a month before my Rhode Island Red( they started laying at 4 1/2 months old), but I have a friend whose Columbian Wyandotte took forever to start laying.
20 weeks or so this time of year. Sex links may start earlier. Most DP breeds will be fro 20-24 weeks as the days get shorter.
Yours will be 5 months old in the middle of December so they may be a little later as those are the shortest days of the year.
You could start them up quicker by adding light on a timer that comes on early morning and off at dawn. The goal is about 12-14 hours of light.