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.
best of luck.
Omran.