You want to get them acclimated to the outdoors asap. Have you let them free range in your yard to start building immunities and experience cooler temps?
You can put them to the coop at any age and any time of year with the proper setup, mainly having a heat source for very young chicks. I've brooded 2 wk old chicks outside in the coop with a
Mama Heating Pad with day temps ranging in the 50-60s and nights in 30-40s. With this type of exposure they feather quickly and no longer need heat by 4 weeks.
If you're not comfortable running an extension cord to the coop for a heating pad, at least during chilly nights, you can build a huddle box or wool hen. That will help them collect body heat and stay warm all night, you just might have to show them how to use it. I wouldn't recommend a heat lamp in the coop. Once they're fully feathered, those feathers are great insulation and will be enough to keep them warm in your climate.