You've got some slow to mature breeds. Brahmas, Cochins, and Ameraucana take about 6 to 8 months to start laying.
One thing about layer feed, if you start them on it too early, it can actually slow their development making it take even longer for them to start laying. And it's my experience that Easter Eggers really need a higher protein feed if you don't want to be waiting till they are 8 months old for eggs.
You never actually NEED to switch to layer feed. There is no magic ingredient in it. It's just regular feed with less protein and a whole lot more calcium. Many actually find that their flocks lay better, and have fewer issues with egg eating and feather picking, if they stick with a higher protein feed like starter/grower than switching to layer.