You can switch them over as they are done with the huge growth sputs they go through when they are younger, but you don't need to. I would keep them on grower until they are all laying, they really seem to appreciate the extra nutrition in the grower.
I have a very mixed flock with 8 roosters and hens ranging from 1 to 6 years old. They all get a 50/50 mix of meat bird crumbles and layer pellet. Oyster shell is always available on the side. Works great for me.
Mrs. Puff, If the medication in Starter is Amprolium there is no withdrawl time, because it is not an antibiotic.
If the medication ends in -cyllin or -cyclin it is an antibiotic and will have some withdrawl time.