Hi Henry - with enough of a heat lamp, you can raise chicks outdoors in just about any temp provided they don't get a cold draft blowing on them. They can always get under or near lamp if they are cold and need room to get away if they are hot. I usually keep mine inside in a brooder till they are a week old just so I can keep an eye on them and make sure all are eating, drinking, pooping. Then into cage in coop with lamp at night or daytime if temps are cool. Ruth's babies are already in coop with no light cause it's sooooo hot and humid here. During day they are allowed to run around coop with the "big girls".
One thing to keep in mind is that with the heat lamp on them, especially while it is already still warm, they don't feather very fast and don't get acclimated to the weather so it's harder to then put them out in cooler weather.
As far as freeranging, mine all have free ranged by 4 weeks but they have also been hanging out with all the adults since one week. I think they learn a lot by hanging out with the older chickens and watching and learning.