my chickens love canning season, they get whatever is left after I have processed stuff...beans, and the bean plants when I pull them out of the garden, cucumbers, tomato peelings/cores raw and cooked ones, apple peelings and scraps, beets and beet tops, though we have been giving those to our steers lately, corn cobs...pretty much anything out of the garden they love, and then sometimes they get table scraps that usually have rice or spagetti or things of that nature...whenever we have about a handful of cereal left in the box, we take it out to them, they love anything that is different then their food it seems...every time I open the back door, they see me and come running to see what I have for them...its funny
Just wanted to add that I never really paid attention specifically to my previously raised "chicks" as to what specific age they started eating "treats" so I dont know that would be up to you how much and how often. I just toss it out in the pen, and they all make a go for whatever is given to them. I know that my chicks I have now havent pecked at just too much by way of people food yet, but they stick close to their mama and stay clear of the frenzy when treats are dispensed...I suppose it wont be long, and they will want a taste too...