Same answer as speckled. Mine were on starter/grower until 19 weeks when I got my first egg. I would have had to travel pretty far to get just medicated starter.
Chicks are most at risk between 3-8weeks old. I don't think switching them during that period is a good idea. Your dropping off the medication and stressing them some through a feed change right when they are most likely to get sick. I switched my first 2 hatches after 10-12weeks and haven't used medicated feed since then.
I haven't ever put mine on starter/grower before but I will start. I always just switched them to layer crumbles slowely and added corn and eventually calcium when they were close to laying. Is it about the same price as the other crumbles? I get the 50 lb pound bags for about 10.