Ours were allowed to play outside since they were four days old, and moved to their coop permanently when they were three weeks old. It's so hot here that it didn't make sense to keep them in any longer, it was warmer outside than in, heat lamp not withstanding. Our brooder is was small for the number of chickens we got (six), so by three weeks they were very ready to enjoy their coop and run.
I think it depends on how big your brooder is, and what the weather is like when they go out. Our coop and run was very safe, much larger than the brooder, and warmer than it too. That's why our chicks got to live outside when they were three weeks old.