I've read several articles focusing on the pecking order. They all say it should be up and running good by about six weeks old. From the book Tending A Small-Scale Flock for Pleasure and Profit by Sue Weaver:
"In any flock of chickens, there are birds who peck at other flock members and birds who submit to other flock members. This order creates a hierarchical chain in which each chicken has a place. The rank of the chicken is dependant upon whom he pecks at and whom he submits to. He ranks lower than those he submits to and higher than those he pecks at.
A flock of chicks generally has their pecking order up and running by the time they are five to seven weeks old.
Pullets and cockerels maintain seperate pecking orders within the same flock, as do hens and adult roosters. Hens automatically accept higher ranking roosters as superiors, but dominant hens give low-ranking cocks and uppity young cockerels a very hard time.
In a closed flock with an established pecking order, there is very little infighting. Each chicken knows his or her place, and except among some roosters, there is suprisingly little jostling for position."
