How Old Does A Baby Chick Need To Be Before Letting It Out On Its Own?

They need to be feathered out well. About ten to twelve weeks should work. If you have older chickens, they may pick on the young ones, so you might need a separate place for the young ones till they get close to the same size as the older chickens.

What do you mean by 'on its own'? If the weather is reasonably warm you can let them out during the day from about 3 days of age, but put them back under a heat lamp during the night. I wouldn't just let them roam free though, as hawks, cats etc will easily get them. I have 2 five day old chicks that go out during the day into a very large aviary to roam free but go back inside under a heat lamp during the night. They are thriving and learning to scratch for food :)