Unless you have bitter cold weather, keeping them inside till 16 weeks is quite extreme. They will let you know when they have had enough of "outside," if they start to shiver a little - time to go back in. Do keep a vigilant eye on them - predators love tasty youngsters, and raptors will notice them also. A covered run is always a good idea, and if a solid type, will also offer some protection from rain / snow, etc.
PS often employees of feed stores and chains - have no education in chicks at all - they may go from stocking shelves, to being a chicken advisor in the same day.
PS often employees of feed stores and chains - have no education in chicks at all - they may go from stocking shelves, to being a chicken advisor in the same day.