My chicks went outside for a few hours on warm days at four weeks old. They were in a chicken tractor though.
Then, when we moved them to their hen house, after locking them inside for a week, they then had free access to their outside, enclosed run every day.
I didn't actually let mine free range (no fencing at all until they were at laying age (around 20 weeks old). Then, the first few times, we stood there and watched them to make sure all was well. If we were working outside we would let them out so we could keep an eye on them.
The older they are the better chance of them not being taken by hawks, etc.