I wait until they are at least two weeks old and they have eaten some chick grit. I pick a day that is at least 60 degrees, sunny, and no wind. At first I only let them out for 15 - 20 minutes. I watch them close to make sure none of hem are getting cold. As they get older I let them out for longer periods of time. They are always supervised. After they have the majority of their feathers I may lock them in the run for a few hours unattended. (The run is secure and has a roof).
I never let any chickens, including adults, free range without supervision. We have too many hawks, eagles, racoons, coyotes, etc. Just a couple of weeks ago a small falcon tried to attack one of my 3 year old birds. It did not cause but surprised everyone. I didn't even know we had falcons in the area.