If we are talking coturnix, at a little over two weeks they are almost completely feathered and you can tell pharaoh male from female with pretty high accuracy. I have been doing serial hatches, so I need the current chicks out of the brooder at about 16 days to give me time to clean it and get it ready for the next hatch. I wean them off of heat gradually (raise the light and then swap out the bulbs for smaller wattage) before I put them out.
It's been getting down to about 50F at night where I am, so my most recent hatchlings will have a shop light with a 60W red bulb in the hidey area of their pen for the first week they are outside (so until 3.5 weeks). No reason not to be cautious, but I put them out without heat if it's in the 60s at night with no problem at all. They are meant to covey up, and mine tend to want to sleep out in the open part of the pen rather than going into the more sheltered area.