Depends on where a person lives. From what I have been able to determine they are sexually mature by the time they are 5 to 6 months old.
Where I am, they start laying in the spring of the year after they were hatched.
In more southern states they may start laying in the fall of the year they hatched.
If they are supplied with supplemental lighting it can throw off their diurnal clock and cause them to lay earlier than normal. Supplemental lighting will not change the fact that they are seasonal layers, it just throws them off of their timeline for starting and stopping laying.