I put mine outside at 5 weeks with a heat lamp. But as long as it's warm outside, you can probably get away with putting them outside at 2 or 3 weeks as long as they have a lamp for heat.
Mine were on an enclosed porch from the start. I kept them in a rubbermaid water tank with a blanket over one side and a screen over the other, a heat lamp in the middle. After about 2 weeks I moved the set up to the barn, eventually blanketing only at night (probably at about 4 wks) and then going to just screening. If it was a particularly cold night I'd cover and turn on the lamp.
My chicks were outside from the very start. As long as you have a draft-free place to keep them where you can keep the temp. at the appropriate level for their age, there's nothing that says you have to have them in your house.