Bet sprouts would be a great first treat, they might think it was a worm. Since they're mostly sater and vitamins, they wouldn't fill them up, either.
There's no set age, just lots of opinions. I've given a bit of treat to brand new chicks, just a few days old, after they'd figured out about their feed and water.
Don't forget to offer a bit of sand or fine grit. I've just set a bit of the fine stuff from the grit bag in there for them.