Two or three weeks old is the earliest to put something like that in the brooder with them. I would recommend something VERY close to the bottom of the brooder, just to get them used to the idea of roosting. They don't need anything more than 3 inches high at first. If you keep them in the brooder until they are 7 or 8 weeks old, you can raise it gradually around 5, or 6 weeks old.
They don't NEED a roost bar, but it's nice to give them one before they go outside full time. And it's adorable to see them roost side by side in the brooder! They probably will not sleep on the roost, but snuggle together in the brooder down on the pine shavings; the roost is mostly to give them that "I'm up high! and I'm a BIG chicken!" feeling.