My understanding has always been that they should be ready to leave the brooder/hen at 4 weeks. The other day I was told the norm is 6 wks. Either my memory is going or there's 2 standards. My hatchery birds have always stayed in the brooder longer, given extenuating circumstances (coop not finished, Numi). This gf batch went out at 4 1/2 weeks. Last year, the ones I incubated, I took outside at 3 weeks and the males refused to give them back, so they went out at 3 wks.
Numi's will be 4 weeks Saturday, and they are constantly on the move, up on the roost, have escaped w/the papas for all day field excursions, I haven't seen them huddle under an adult or together at all this week. They're in the thick of things, using their wings. I just looked out there and they're on the roost in the window, 70° out. I do shut the fan off at night, probably ought to go shut that light off.

(It was threatening rain & dark in the coop early so I left a solar light on so they could eat.)
Neither Mama's nor Numi's hatches were medicated with anything, no vitamins, just startena. Gf's were tx w/corrid and vt b complex (@diff times),leaving 7/20 over the course of one wk. Hoovers didn't get that far, all deaths were within 48 hrs,leaving 7 out of 44.
I've not seen my hatchery birds be unkind to Numi's keets, at one time I think they wanted to jump in and Nanny, but she wasn't having it, only let the jumbos near. Now her keets are under the feet and eating with the gfs- no fear.