As 100 poultry keepers this question and you'll get 101 answers, lol.
Really, there is no set rule, timeline, etc at which you must make this change or that -- other than do not provide a bird the excessive calcium of layer ration until that bird actually needs the calcium (meaning is producing, shelling and expelling eggs) as the excess calcium can actually do more harm than good prior to it being needed. IF one chooses to feed layer, make that transition from starter or grower at the time the birds begin to lay. Really , it's about finding what works for you/your birds. Starter is simply a very small/fine form so that it is easy for day-olds to consume, grower is a bit coarser (and sometimes has a slightly different protein content) than starter for older chicks - likewise with finisher, often minor tweaks to the nutritional breakdown and possible to the form of the feed as well. The differences, nutritionally, between the three are negligible imo.
That being said, you can actually feed a grower ration for the entire lifespan of your birds - as an example, I use Flock Raiser. Every bird I have, from new babies to established layers, eats this ration. I simply provide calcium in the form of free choice oyster shell for the actively laying birds to take as they want/need.