15 or 16 weeks is absurd. New chicks need extra heat and protection til they are fully feathered, which is usually around 5 or 6 weeks. They can be outdoors from day one, if you are set up to brood chicks outdoors, as many of us are, often after going through brooding them indoors. The only reason that I can think of to keep them indoors longer than 5 or 6 weeks is predator protection, since they are not full size til around that age. Actually, they do not reach mature weight and "heft" wil around one year, but reach adult height around 16 weeks.
The people at my feed store will tell you that cracked corn is chicken feed. They sell grower, etc. but you have to ask specifically for a formula. Cracked corn has less than half they protein they need, and essentialyl no vitamins and minerals. Feed store employees often are not trained in chicken care, and are rather well known here for giving out incorrect advice (and kudos to those who DO know what they are talking about!)