it really depends also on how worm the weather where you live!!
if you live in very worm area where tempratures don't go below 50 at night then you can put them earlier outside in thier coop if the coop is draft free and well insulated, and you still have to put the heat lamp for them in thier coop, so if they feel cold they go undernieth it and worm up.
my chicks are 14 days old and I am planning on put them in thier coop next week.
I put my EE's outside for the first time last weekend and they weren't even 4 weeks old yet. I had a heat lamp on 24/7 in the coop though. Yesterday I started turning it off in the morning since it gets into the mid 60's in the afternoon, but it still goes on in the evenings because it's in the low 40's at night here.
Chicks can go out to a coop that is draft free and predator proof at any age, as long as you can maintain the coop/brooder at the temperature needed per their age. 90 to 95 degrees the first week, decreasing by 5 degrees per week.
My chicks were brooded outside.