I start feeding mine treats around a week. As long as they have grit, and you don't give them too many it's fine
Depends on your definition of 'treats'. I started feeding my chicks grit at week two, and introduced them to grass and mealie worms. The worms they went absolutely nuts for. The biggest thing to remember is that you have to provide grit to them. Without the grit, they won't be able to break down anything except the crumble. At the age of your chicks, mine were eating meal worms (live and dead) and crickets occasionally, the crumble, some veggies from my fridge that were starting to get past what we would eat, but not bad. They also got dandelions from the lawn and grass. Since then I've introduced apples, which they haven't gotten the idea of yet.. but I know they will, and some scratch in the evenings.
I got chicks recently and I started to feed them meal worms a little before four weeks old. They started to get visits outside maybe the day before they turned three weeks old.
My 5 week old chicks loved corn, lettuce, blueberries, watermelon, peas, swiss chard and other treats that I had around the house. They were also free range to get all the bugs around the barn.
Heck I was feeding mine treats at 2 days old! As long as the treats are healthy, and they have grit, I see no problem. I feed mine meal worms, bits of cheese, greens and weeds, bits of fruit, and yogurt. I consider bread, rice, pasta, etc.. to be "junk food", empty calories. They are so spoiled they go nuts when they see me!