I try to develop chicks into 'good greens eaters'. I start their first 'treats' at 2 weeks old by clipping grass or clover into the brooder. I use scissors to snip it to baby size. In cold weather I use snipped lettuces. Also start providing chick grit at this time.
I only feed greens as treats until they're about 6-8 weeks. Then I like to start them on cooked carrot or sweet potato, raw pumpkin, squash or zucchini, occassionally a little fruit.
I don't introduce anything starchy like corn, bread, rice, crackers at all until they're about 3 - 4 months. Starchy stuff is candy in the chicken world. I feel introducing these last makes better layers, not picky eaters who wait to gobble up carbs. They forage better as free range hens and readily eat greens or orange veggies - resulting in richer, darker yolks.