The rule of thumb I was taught some 30 years ago and have adopted ever since was a minimum of 8 weeks on starter, then move to a grower crumble till they are big enough for pellets. For layers I switch to layer pellets at 20 weeks, to give them a good solid couple weeks with the higher calcium and minerals.
After 8 weeks they do not need the high protien content as found in starter and the crumbles you will not find in a starter. You will find they will waste less with a crumble mix, and pellets they can pick up and eat later as it is spilled on the ground.
In addition, grower cost less and they are old enough to eat scratch, greens and other fiid sources.
Not saying its the right way, just one way that I was taught and have used with great success.