If you mean supervised outside or inside a predator proof run, I started taking ours out at five days for 15-20 minutes at a time, and would have done sooner had it not been raining those first five days. If they had a mama hen they'd go out around a day or two old.
If you mean running around the yard with no run or supervision - beware of crows & cats.
Our chicks are four going on five weeks old, and I've been taking them outside for longer and longer periods, getting them ready to be outside full time. With the nice weather the last few days, I've left them in their outside coop/run with a heatlamp available (which they never went under) during the day, and I bring them in at night.
They will be outside full time by six weeks (assuming the weather cooperates and they are fully feathered.)