mine are 5 weeks old and just starting to be interested in treats. I've read here about all the different stuff chickens like. I'm sure eventually they'll like it all, but they started in phases so here is what I discovered:
at 3 weeks I tried giving them yogurt. They weren't interested.
at 4 weeks, I tried giving them a slice of tomato. They pecked at it sort of bored-like. The next day I gave them a piece of cheddar cheese and they went NUTS.
at 4.5 weeks I stuck a piece of celery through the wire and they slowly pecked at it until it was gone.
last night, they wolfed down some lettuce. Also, I caught a tiny grasshopper (the size of my thumbnail) and tossed that in. It disappeared as soon as it hit the floor!
I've noticed that when I take them outside, they peck at the fluffy dandelions. Hmm, what should I try next??