In my experience, as long as they have some place warm, dry and safe to get to they will be fine for some outside play on sunny days. Mine are usually raised with a hen, but I have had several that have been abandoned around 3 weeks to make their own way amongst the flock. Two of these 'orphans' are still in my flock; one is now a beautiful 23 week old rooster, the other is a 5 week old hen that has done just fine in temps as low as the 20s free ranging during the day.
That said, chicks and chickens need to acclimate to varied temperatures and always have a haven to warm up if need be. They aren't nearly as fragile as one might think, but if put out suddenly or without proper shelters they risk sickness and death.