Chickens can free range at almost any age. Of course when they're little chicks, I don't recommend that because they can be taken by a predator so easily. What you can do is, at first, let them out about an hour or two before dark. You'll be surprised, because when it gets dark, they'll go right back into their coop for the night. (This only works if they're very used to their coop and consider it their home though.)
I like to wait until they're fully feathered, and they don't look like babies anymore before I let them free range. This is because the smaller they are, the more vulnerable they are to hawks and such.
Welcome to BYC. At five weeks we moved our flock into their full time coop. I believe I left them in for about three or four days so they knew it was "home" and from then on they have free ranged. Chicmom is correct: when the sun goes down the chickens go to roost. But at first they may need some assistance. For a few weeks I had to lead them in because they wanted to roost on our porch rails. I also had to put them on their own roosts for a few days. They would sleep on the floor at first. But once I plopped them on they figured out all they felt they needed to know and do their thing.
Ours free range as soon as they don't need a heat lamp. We have electric netting - and while some could go right through it if they wanted, they usually don't.
They are so susceptible to predators early on. If they listen to warning calls from roosters - they can free range.. We always keep a rooster as a babysitter for chicks.
(this was in the peak of summer, where it was too hot to use a heat lamp. Chicks went out at 2 weeks).
This is a pic of our flock. 2 Americaunas (Chicklet and Ditto), 1 Rhode Island Red (Betty), 1 Lavender Pekun (sp) (Lucy) and the rooster is a Barred Rock (Axel). This pic shows their fenced in area off of their coop, where they have been kept so far.
I let mine out at around 20 weeks... and that was after about 8 weeks in their coop and run. One day they just seemed more interested in exploring the outside world, so I left their run door open and they slowly made their way outside. Hubby thought I was nuts and was sure we'd never see them again. They stayed within 10 feet of the coop the first day and returned inside when we brought out their dinner treats. They are supervised at all times due to predators... in fact, a hawk just swooped into a tree about 10 feet from me trying to get at the song birds. I yelled hawk and it took off but predators can be very bold when hungry.
I let mine out to range at around 3 months. At first is was just for a few hours when I was out in the yard and could keep an eye on them. I slowly let them out more and more as I got more comfortable with the idea of them being free. They seem pretty predator aware even without a rooster to guard them. Eagles were my biggest concern but we have lots of cover, most of our lot is forested and there are lots of brush and understory bushes for the chickens to hide under. Anything at all flies over and they all run for cover. They stay close enough not to cause problems with the neighbors. We have a fairly large lot but its not fenced so I wanted to make sure the chickens didn't wonder too far. After a few weeks of watching and supervising their time out I pretty much let them free range most of the day now.