Chicks will usually start to try to jump out of their brooder at about one to two weeks of age. You can choose to move them to a bigger brooder at that point, or to cover their brooder with a hardware mesh screen (1/2 inch mesh, not one inch!). Today, one of my week old Marans poked her little head above her brooder, which is just a galvanized wash tub, one foot high. She was standing on the bunny I'd given her for company. I took the bunny away because now she has three more Marans for company and because I don't want her hopping out just yet!
You'll want to put your nestboxes and your roosts at a height good for the breed of chicken you have. Some heavy breeds, like Cochins, would become Cochin pancakes trying to get to roosts higher than a a foot off the ground. Other breeds, like Seabrights and Old English Game, will fly as high as you can imagine (they'd happily roost in trees). You need to find a happy medium between what is good for your chickens and what is not inconvenient for you.
As for putting them outside, eight weeks is really the earliest and only, as PPs have said, when fully feathered. If you live in a temperate or cool climate, you will want a heat source in your coop to bring the ambient temp to about 72 degrees for them... and you'll want to maintain that temperature year round. If you're in a warmer climate, you'll want lots of additional ventilation for your coop to keep it from becoming a chicken roaster. Either way, make sure there's LOTS LOTS LOTS of water for them to drink. And as a PP noted, you'll need a pop door and ramp for your chickens to allow them access to their run. And make sure that pop door latches up firmly and securely, so that raccoons don't help themselves to a free chicken dinner!