If you live in Phoenix, they can be outside as soon as they are fully feathered (5 weeks)... but mine usually still get the heat lamp until they are 8 weeks old.
If you live in cold climes, they need to be a little older or they need to have a well placed lamp or bulb in their coop.
what type of coop do you have ? Is it insulated? What breed ? You should wait till they are fully feathered. I put mine out at 6 weeks, this was late fall in ny. Up by you it must be a bit colder so you want to wait a bit longer if you can. You may want to keep some heat on them letting it get cooler as the days go by (hardening off, aclimating) so they can get used to the cold. Theres some more info here https://www.backyardchickens.com/web/viewblog.php?id=1642-winter-coop-temperatures