This is going to sound complicated because there are a whole lot of options that work. It is actually extremely simple because so many different things work. I'm only talking about feeding chicks that are going to be a laying flock. If you are raising meat birds, then it is different. They need a higher protein level.
The normal progression is to feed them Starter (around 22% protein, but it varies) for the first 4 to 8 weeks, then switch to 16% protein Grower until they are 20 weeks old or they start to lay. Then you switch to 16% protein Layer. Layer has a higher percentage of calcium in it for the egg shells. But a steady diet of too much calcium for a growing chick can cause bone deformation or kidney damage. About the only hard and fast rule in this is to not feed Layer to growing chicks. The protein is the major difference in the different types of feed, other than the calcium in the Layer.
Different manufacturers have different recommendations as to when is best to switch from Starter to Grower, anywhere from 4 to 8 weeks. In actual practice, it does not matter. When you run out of Starter after 4 weeks, switch to Grower. Even if that is 10 oe 15 weeks, you'll be OK.
Some feed stores do not have separate 22% Starter or 16% Grower. Instead, they provide an 18% or 20% combined Starter/Grower. You can feed them this from the day they go into the brooder until you switch to Layer.
Some manufacturers offer a 15% Grower/Developer. This works well for the chicks from 13 weeks until you switch to Layer.
Another option is to feed a 20% Flock Raiser. This is pretty similar to the combined Starter/Grower. This is intended for flocks where some of the chicks are destined to be a laying flock and some are intended to be harvested for meat, sort of a compromise between the higher protein for meat birds and the lower protein for a flock that will be layers.
I personally like to feed the 16% Grower or 15% Grower/Developer to the adolescents on the thought that it slows their body development to better match the maturity rates of their internal organs and skeleton. But that is pure personal preference. I often wind up feeding the combined Starter/Grower because that is the only choice available at my feed store. Lots and lots of people feed the higher protein feeds and do great. I do recommend staying away from the really high protein mixes that are intended for game birds. Too much protein can cause problems, but that boundary for too much is pretty wide. You'll be OK with any that I mentioned.
When they start to lay, you do not have to switch to Layer. You can continue with Grower, Flock Raiser, or some other things and just offer oyster shell on the side. This is what I do when I have mixed ages in my flock.
For those that do not believe the extra calcium in layer can harm chicks, you might read these.
Avian Gout
http://en.engormix.com/MA-poultry-i.../avian-gout-causes-treatment-t1246/165-p0.htm
British Study Calcium and Protein
http://www.2ndchance.info/goutGuoHighProtein+Ca.pdf
