I find that at maybe 2-3 weeks old I start introducing yard "weeds" (look up what is not toxic) [chickweed and bedstraw/catchweed, stinging nettles (wear gloves!) wild clover and grass in my area of Texas is what I give] to my feed store bought chicks. I provide chick grit in a container near food and I start introducing a shallow cardboard box filled with clean yard dirt, contractor sand (so it has grit in it), "little" diatomaceous earth and powdered lime (First Saturday brand) and some dried herbs to ward off bugs (lice, fleas, ticks, mites) so they can clean their feathers and destress. I also introduce BSF (Black Soldier Fly larvae) to them, cooled scrambled eggs (no butter), tiny pieces of fruit, pieces of cucumber or squash. Of course once they have some feathers and the weather permitting, take them out in a secured run (that you watch over them) to scratch, eat and be happy little chicks!