My chickens are 13/14 weeks and they have been eating small amounts of scratch for 4 weeks now.
For my chickens, scratch is a treat, as suggested by BYCers.
My chickens diet:
scratch 1%
fruits, veggies, yogurt, etc 9%
chick crumbles 90%
They don't get out to free range much, so bugs are <<1%
My chickens actually love everything else in the scratch mix except the cracked corn, which they do eat after all the other stuff is gone. They eat right out of my hands (very small handfuls).
My recommendation would be to buy a scratch mix that is more than just cracked corn. Older chickens might like the cracked corn, but my birds like the other seeds in the mix.
--Hugh