What matters most is that the chicks have been exposed to the world while still consuming the medicated feed for about a month. The medicated feed helps them build resistance to cocci and if they haven't been exposed, the medicated feed doesn't do them any good. I usually feed them the medicated feed until they are 8 weeks old given that they have been put out into a grow-out pen around the age of 5 weeks.
Once I forgot the reason for the feed and stopped feeding it at 8 weeks, just like always, but I was keeping them in a ginormous brooder in the shed longer than that; they were 10 weeks old when I finally got their grow out coop and pen set up. By then they had been on regular feed for two weeks..... and I lost about 10 of them in a cocci outbreak. It had been raining and the soil was very wet and muddy, perfect environment for cocci to flourish. My adult birds were resistant, but the youngsters weren't.
Just sayin'.....