I've been curious myself about the age at which a chicken achieves their full growth. So I used the handy "search" function at the top of this page.
I found that the age the chicken is fully grown depends on the breed. They say that Brahmas aren't at their full weight and size until age two. Others are fully grown some time after one year.
As for when you can expect your girls to start laying, that also depends on the breed. Some are slow to mature, and others, like Wyandottes, are ready to go around five months. Watch their combs. When they begin to turn bright red, the first little starter eggs aren't far off. Also, if your pullet squats when you reach to pick her up, that's another sign they're getting very close to producing that first egg!