2 Answers

Most hens start to lay around 4-6 months of age, more or less.However, for various reasons (time of year, temperature, etc.) they may wait until the next Spring to start laying eggs.

http://www.backyardchickens.com/a/chicken-faqs-the-frequent-asked-questions-of-raising-chickens

Most breeds will lay at aprox. 21 weeks of age.  Hens lay because it is their reproductive cell that they produce.  Like all birds, they do this with an environmental cue.  That is daylight hours, think of spring, and the days are getting longer, so that is what begins their reproductive activity to lay eggs.  Now if we "fool" their brain into thinking it is "spring" all year, by manipulating the light, with artificial lights in their pen, even in winter when days are short, we control their egg laying to meet our food needs.  It takes about 25 1/4 hrs. for an egg to be assembled in her body, so it seems that she takes a day off about every couple of weeks, but it is only being laid later within each 24 hr. day, each day...  And another thing, they DO NOT need roosters to lay eggs, only to lay fertile eggs.  Think of a woman, she ovulates monthly, and  doesn't need a male for that!  Only to fertilize that ovum.  Good luck...