Depends on the breed and the individual bird.
One of my LF brahma pullets started laying at 19 weeks old (almost 5 months old). The last two of my brahma pullets to lay were 38 weeks old (9 & 1/2 months old).
All raised together since hatch. Just goes to show chickens are individuals, just like the rest of us.
My first heavy breed chicken started laying at 17 weeks most of the rest between 20 and 23 weeks. The lastest was 27 weeks. I recently had one of my pullets start laying and she was 17 weeks also. My pullets are 20 weeks. My older girls are 2 yrs old. I think in another day or two, more pullets will start to lay. They are showing all of the signs.