You can keep them on starter for their whole lives but when they get to be about 8 weeks old I'd start mixing in some scratch or birdseed (be sure to offer grit or see that they are free ranging for grit) to get your protein levels down some to 17% or so. This is hailed as important to prevent reproductive problems, I have read. There are of course people who keep them on 20% protein their whole lives but I like to have it a bit lower.
Your starter is likely 20% protein, and millet in birdseed is 11%, black oil sun. seeds are 16%. Corn is 7-9% and wheat is around 12-14% usually.
You can switch them to layer when they are around 20 weeks of age or when they start laying. Put some oyster shell out for them around that time, even if you start them on layer feed.
Layer feed should never be fed to chicks due to the high calcium. It isn't good for roosters either. You can just keep them on unmedicated chick starter or some other high protein feed forever, giving some other mixed grains/seeds (make sure they are getting greens such as grass or alfalfa/vegetable scraps year-round to prevent vitamin deficiencies) to round out the diet, if you wish.
When they are at the point of lay I like to have the protein level around 16% if possible. That
is usually what layer feed is.
Enjoy your chickens!