It's best to feed them chick starter until they are old enough to go out the coop (8 weeks is when I Usually put them out) This way you don't have to feed the little ones grit To help break down any food they can not digest, this is why they have chick starter.
Most scratch has corn as the primary ingrediant is not good for any chickens if fed to them very often. It is basically chicken "candy". I admit that I give my full grown birds a very small amount of scratch in the winter but only a little bit. This year I bought a bag of it in October. Occasionally I would toss 2 cups of it in the run for 30 birds. They would eat all of it in less than 5 minutes. I still have about 1/4 of the bag remaining. BOSS (black oil sunflower seeds) is the primary treat they get. When I have new chicks they don't get their first taste of scratch until they are at least 16 weeks old. And then it i in VERY SMALL amounts.
I have to admit, I spoil my chickens. They're pets so whenever I fill their feeder, half of it's scratch. But on the bright side, My leghorn, Meggy laid 2 extra eggs, and my other girls each laid 1 extra egg!
I have given 3 week olds a tiny bit of scratch. They love uncooked oatmeal too. I also gather a handful of grass and cut it in tiny pieces with sharp scissors...that's their favorite.